Republic Act No. 9522, which adjusts the Philippines' archipelagic baselines and classifies the Kalayaan Island Group and Scarborough Shoal as "regimes of islands" in accordance with UNCLOS III, is constitutional; it serves as a statutory mechanism for demarcating the country's maritime zones and does not result in the diminution of Philippine territory or sovereignty.

The Philippines, an archipelagic state, enacted Republic Act No. 3046 in 1961 to define its maritime baselines. Following its ratification of the United Nations Convention on the Law of the Sea (UNCLOS III) in 1984, there arose a need to align domestic law with the Convention's specific requirements for archipelagic baselines, such as water-land ratio, maximum length of baselines, and conformity to the general configuration of the archipelago. UNCLOS III also established a deadline for coastal states to submit claims for an extended continental shelf. Consequently, Congress enacted RA 9522 in March 2009, amending RA 3046 to comply with UNCLOS III by optimizing basepoints, adjusting baseline lengths, and classifying the Kalayaan Island Group (KIG) and Scarborough Shoal as "regimes of islands" under the Republic, whose islands would generate their own maritime zones distinct from the main archipelago's baselines.

• In 1961, Congress enacted Republic Act No. 3046, demarcating the maritime baselines of the Philippines. This was later amended by Republic Act No. 5446 in 1968 to correct typographical errors and reserve the drawing of baselines around Sabah.
• To comply with the United Nations Convention on the Law of the Sea (UNCLOS III), which the Philippines ratified in 1984, Congress passed Republic Act No. 9522 in March 2009.
• RA 9522 amended RA 3046 by shortening one baseline, optimizing the location of some basepoints around the Philippine archipelago, and classifying the Kalayaan Island Group (KIG) and the Scarborough Shoal as "regimes of islands" under the Republic, whose islands generate their own applicable maritime zones and are situated outside the main archipelagic baselines.
• Petitioners, consisting of professors of law, law students, and a legislator, initiated an original action before the Supreme Court, seeking writs of certiorari and prohibition to declare RA 9522 unconstitutional.

• RA 9522 is unconstitutional because it reduces Philippine maritime territory and diminishes the reach of Philippine sovereign power, contravening Article I of the 1987 Constitution, which petitioners argue embodies the territorial definition derived from the Treaty of Paris and related treaties.
• The law unconstitutionally converts internal waters into archipelagic waters, thereby subjecting these waters to the right of innocent and sea lanes passage by foreign vessels and aircraft, which undermines Philippine sovereignty, national security, the country's nuclear-free policy, and damages marine resources.
• The classification of the Kalayaan Island Group (KIG) and Scarborough Shoal as "regimes of islands" outside the main archipelagic baselines results in the Philippines losing approximately 15,000 square nautical miles of territorial waters and prejudices the livelihood of Filipino subsistence fishermen.
• RA 9522 is facially unconstitutional due to its failure to reference the Treaty of Paris or Sabah, and for its adoption of UNCLOS III's "regime of islands" framework to determine the maritime zones of the KIG and Scarborough Shoal.

• Petitioners lack locus standi to file the suit, and the writs of certiorari and prohibition are not the proper remedies to challenge the constitutionality of RA 9522.
• RA 9522 is constitutional as it represents the Philippines' compliance with its obligations under UNCLOS III, and it preserves Philippine territory, including its claims over the KIG and Scarborough Shoal.
• The enactment of RA 9522 does not undermine the country's security, environmental and economic interests, nor does it relinquish the Philippines' claim over Sabah.
• Petitioners' assertion that the Treaty of Paris defined Philippine territory to include all waters within the rectangular area described therein lacks normative force under international law.

• Whether the petitioners possess locus standi to bring the suit.
• Whether the writs of certiorari and prohibition are the proper remedies to assail the constitutionality of RA 9522.
• Whether Republic Act No. 9522 is unconstitutional on the grounds that it allegedly reduces Philippine maritime territory, undermines national sovereignty, and violates various constitutional provisions.

• Petitioners possess locus standi as citizens with a constitutionally sufficient interest in the resolution of the case, which raises issues of national significance, making it difficult to find other litigants with a "more direct and specific interest."
• The writs of certiorari and prohibition are traditionally viewed by the Supreme Court as proper remedial vehicles to test the constitutionality of statutes when exercising its power of judicial review, especially for issues of constitutional import.
• Republic Act No. 9522 is not unconstitutional. It is a statutory tool designed to demarcate the country's maritime zones (territorial sea, contiguous zone, exclusive economic zone) and continental shelf in compliance with UNCLOS III, not to delineate or acquire/lose Philippine territory itself.
• UNCLOS III primarily regulates sea-use rights over maritime zones and does not govern the acquisition or loss of territory, which is determined by general international law principles like occupation, accretion, cession, and prescription.
• The classification of the Kalayaan Island Group (KIG) and Scarborough Shoal as "regimes of islands" under RA 9522, generating their own maritime zones, is consistent with Article 121 of UNCLOS III and does not constitute a renunciation of Philippine sovereignty over these areas; Section 2 of RA 9522 explicitly affirms continued Philippine sovereignty and jurisdiction. Enclosing them within the main archipelagic baselines would violate UNCLOS III's rules on baseline configuration and length.
• Contrary to petitioners' claims of territorial loss, RA 9522, by optimizing basepoints, actually increased the Philippines' total maritime space (internal waters, territorial sea, and exclusive economic zone) by 145,216 square nautical miles compared to the previous baseline law, RA 3046.
• RA 9522 did not repeal Section 2 of RA 5446, which pertains to the delineation of baselines around Sabah, thus keeping the claim open.
• While UNCLOS III subjects archipelagic waters (which include waters previously considered internal under the Constitution) to rights of innocent passage and archipelagic sea lanes passage, this is a recognized feature of international law and a concession by archipelagic states in exchange for the international recognition of their archipelagic status and sovereignty over these waters. These passage rights are part of customary international law and are automatically incorporated into Philippine law.
• The constitutional provisions in Article II (Declaration of Principles and State Policies) invoked by petitioners are generally non-executory and serve as legislative guides; petitioners failed to provide a sufficient factual basis to substantiate any claimed violation of these provisions or other executory constitutional rights by RA 9522.

• Locus Standi (Citizenship Standing) — The legal right of a party to bring a lawsuit, requiring a personal and substantial interest in the outcome of the case. The Court granted petitioners locus standi as citizens due to the case's "national significance" and the difficulty of finding other litigants with a "more direct and specific interest" in challenging RA 9522.
• Judicial Review (via Certiorari and Prohibition) — The authority of the judiciary to examine and determine the constitutionality of laws and governmental actions. The Court affirmed that writs of certiorari and prohibition are "proper remedial vehicles to test the constitutionality of statutes," even if strict procedural adherence is lacking, when issues of "constitutional import" are raised.
• Archipelagic Baselines (UNCLOS III) — Straight lines joining the outermost points of the outermost islands and drying reefs of an archipelago, used as starting points to measure the breadth of maritime zones (territorial sea, contiguous zone, exclusive economic zone, continental shelf). RA 9522 was held to be a statutory mechanism for establishing these baselines in compliance with UNCLOS III, not for defining or altering Philippine territory.
• Regime of Islands (UNCLOS III, Article 121) — A provision in UNCLOS III stating that naturally formed areas of land, surrounded by water and above water at high tide, are entitled to their own territorial sea, contiguous zone, exclusive economic zone, and continental shelf, unless they are rocks unable to sustain human habitation or economic life. RA 9522's classification of the KIG and Scarborough Shoal under this regime was deemed consistent with UNCLOS III and did not imply a loss of sovereignty, as these islands generate their own maritime zones.
• Pacta Sunt Servanda — A fundamental principle of international law requiring states to honor their treaty obligations in good faith. The Court noted that Congress's decision to classify the KIG and Scarborough Shoal according to UNCLOS III's framework (Article 121) demonstrated the Philippines' "responsible observance of its pacta sunt servanda obligation under UNCLOS III."
• Sovereignty over Archipelagic Waters (UNCLOS III, Article 49) — The principle that an archipelagic State exercises sovereignty over the waters enclosed by its archipelagic baselines (archipelagic waters), the airspace above them, and their bed and subsoil, and the resources contained therein. UNCLOS III affirms this sovereignty, even while providing for certain passage rights.
• Innocent Passage and Archipelagic Sea Lanes Passage (UNCLOS III, Articles 52 & 53) — Rights granted under international law for foreign ships (and aircraft, in the case of sea lanes) to traverse an archipelagic state's waters for continuous and expeditious transit, subject to specific conditions. The Court recognized these as customary international law, automatically incorporated into Philippine law, and noted that their imposition under UNCLOS III was a "concession by archipelagic States, in exchange for their right to claim all the waters landward of their baselines...as archipelagic waters subject to their territorial sovereignty."
• Incorporation Clause (1987 Constitution, Article II, Section 2) — A constitutional provision declaring that the Philippines adopts the generally accepted principles of international law as part of the law of the land. This was applied by recognizing customary international law principles like innocent passage as operative within Philippine law.
• Non-Self-Executing Constitutional Provisions (1987 Constitution, Article II) — Declarations of principles and state policies that generally serve as guides for the legislative and executive branches and are not judicially enforceable as direct sources of rights without enabling legislation (with some exceptions like the right to a balanced ecology). The Court found that petitioners' reliance on several Article II provisions to invalidate RA 9522 was misplaced as these are "mere legislative guides" and the alleged violations lacked factual substantiation.

• "UNCLOS III has nothing to do with the acquisition (or loss) of territory. It is a multilateral treaty regulating, among others, sea-use rights over maritime zones (i.e., the territorial waters [12 nautical miles from the baselines], contiguous zone [24 nautical miles from the baselines], exclusive economic zone [200 nautical miles from the baselines]), and continental shelves that UNCLOS III delimits."
• "Baselines laws such as RA 9522 are enacted by UNCLOS III States parties to mark-out specific basepoints along their coasts from which baselines are drawn, either straight or contoured, to serve as geographic starting points to measure the breadth of the maritime zones and continental shelf."
• "RA 9522’s use of the framework of regime of islands to determine the maritime zones of the KIG and the Scarborough Shoal, not inconsistent with the Philippines’ claim of sovereignty over these areas."
• "The enactment of UNCLOS III compliant baselines law for the Philippine archipelago and adjacent areas, as embodied in RA 9522, allows an internationally-recognized delimitation of the breadth of the Philippines’ maritime zones and continental shelf. RA 9522 is therefore a most vital step on the part of the Philippines in safeguarding its maritime zones, consistent with the Constitution and our national interest."

• Kilosbayan, Inc. v. Morato — Referenced in the context of locus standi, particularly the requirement for a litigant to possess a direct and specific interest, though the Court ultimately granted petitioners citizen standing.
• Pascual v. Secretary of Public Works — Cited as an instance where taxpayer standing was recognized, relevant to the broader discussion on locus standi.
• Sanidad v. COMELEC — Also cited as an example of recognized taxpayer standing, forming part of the Court's threshold issue analysis on locus standi.
• Francisco, Jr. v. House of Representatives — Its criteria for granting citizenship standing were found applicable to the petitioners, justifying their locus standi on grounds of national significance.
• Aquino III v. COMELEC — Used as an example where a petition for certiorari and prohibition assailing a law's constitutionality was decided on its merits, supporting the propriety of these remedies for constitutional challenges.
• Aldaba v. COMELEC — Referenced as a case where the Supreme Court issued a writ of prohibition to declare a law unconstitutional, affirming the remedy's appropriateness.
• Macalintal v. COMELEC — Cited as precedent for using writs of certiorari and prohibition to declare portions of a law unconstitutional, thus supporting their use for judicial review of statutes.
• Neri v. Senate Committee on Accountability of Public Officers and Investigations — Illustrates the Court's power to issue a writ of certiorari against actions of other government branches, pertinent to the scope of judicial review.
• Oposa v. Factoran — Mentioned as a case that treated the right to a healthful and balanced ecology (Article II, Section 16 of the Constitution) as an exception to the general rule that Article II provisions are non-self-executing, though the Court found no such substantiated claim in the present case.

• Republic Act No. 9522 — The statute whose constitutionality was assailed, enacted to amend Philippine archipelagic baselines in compliance with UNCLOS III.
• Republic Act No. 3046 — The original Baselines Law of the Philippines, which RA 9522 amended.
• Republic Act No. 5446 — An amendment to RA 3046, particularly its Section 2 (reserving the delineation of baselines for Sabah), which the Court found was not repealed by RA 9522.
• United Nations Convention on the Law of the Sea (UNCLOS III) — The primary international treaty governing the law of the sea, compliance with which prompted RA 9522. Key articles referenced include:
  • Art. 47 (Archipelagic baselines): Provided the rules (e.g., water-land ratio, length limits, general configuration) that RA 9522 aimed to satisfy.
  • Art. 48 (Measurement of maritime zones from baselines): Establishes baselines as the starting point for measuring maritime zones.
  • Art. 49 (Legal status of archipelagic waters): Affirms the sovereignty of the archipelagic State over waters within its baselines.
  • Art. 121 (Regime of islands): Provided the framework for classifying KIG and Scarborough Shoal, allowing them to generate their own maritime zones.
  • Arts. 52 & 53 (Rights of innocent passage and archipelagic sea lanes passage): Define passage rights through archipelagic waters.
• 1987 Constitution, Article I (National Territory) — Petitioners claimed RA 9522 violated this by reducing territory. The Court ruled RA 9522 demarcates maritime zones, not territory.
• 1987 Constitution, Article II (Declaration of Principles and State Policies) — Specifically Sections 2 (incorporation of international law), 7 (independent foreign policy), 8 (freedom from nuclear weapons), and 16 (balanced and healthful ecology), which petitioners alleged RA 9522 violated. The Court found these generally non-self-executing or the claims unsubstantiated.
• 1987 Constitution, Article XII, Section 2, paragraph 2 (National Economy and Patrimony) — Concerning protection of marine wealth; the Court found RA 9522 aids in delimiting zones for such protection.
• 1987 Constitution, Article XIII, Section 7 (Social Justice and Human Rights) — Concerning protection of subsistence fishermen; the Court found no prejudice from RA 9522.
• Treaty of Paris (1898) — Historical treaty ceding the Philippines to the US. Petitioners' argument that its metes and bounds defined inviolable Philippine territory (including waters) was deemed unpersuasive against UNCLOS III's framework.

• Justice Presbitero J. Velasco, Jr. — Concurred with the ponencia, adding that RA 9522, as a baseline law implementing UNCLOS III, does not surrender territory but rather defines sea limits for exercising sea-use rights. He emphasized that the constitutional definition of national territory is broad enough to encompass RA 9522's baselines. Justice Velasco also stressed that the Philippines maintains sovereignty over the KIG and Scarborough Shoal despite their classification as "regimes of islands." He reiterated the Philippine position, consistent with its Declaration upon signing UNCLOS III and the 1987 Constitution, that waters within its baselines are considered internal waters, where passage by foreign ships is not an inherent right but a privilege that may be granted by the Philippines, and not automatically conferred by UNCLOS III in the same way as archipelagic waters passage.