The Senate Blue Ribbon Committee is enjoined from compelling the petitioners to testify and produce evidence because its contemplated inquiry is not in aid of legislation and, if pursued, would violate the principle of separation of powers by encroaching upon matters already within the jurisdiction of the Sandiganbayan, and would disregard the petitioners' constitutional rights, particularly the right against self-incrimination, given the pending judicial case involving the same issues.

Following the 1986 People's Revolution, the Presidential Commission on Good Government (PCGG) was tasked with recovering alleged ill-gotten wealth accumulated by former President Ferdinand E. Marcos, his family, and close associates. One such associate was Benjamin "Kokoy" Romualdez. The PCGG filed Civil Case No. 0035 before the Sandiganbayan against Romualdez and others, including the petitioners, for reconveyance, reversion, accounting, restitution, and damages related to numerous corporations allegedly acquired through illicit means. The Senate Blue Ribbon Committee subsequently initiated an inquiry into the purported sale of these Romualdez corporations to the Lopa Group, a transaction also central to the PCGG's case.

• On July 30, 1987, the PCGG filed Civil Case No. 0035 with the Sandiganbayan against Benjamin "Kokoy" Romualdez, his wife, Ferdinand and Imelda Marcos, and others for reconveyance, reversion, accounting, restitution, and damages.
• The complaint was amended, impleading the petitioners as party defendants, alleging their involvement in schemes to unjustly enrich the Romualdezes, including the manipulation and concealment of assets and the purported sale of 36 Romualdez corporations to PNI Holdings, Inc. (controlled by Bengzon's law firm) for P5 million to deceive the PCGG.
• On September 13, 1988, Senator Juan Ponce Enrile delivered a privilege speech alleging a "take-over" of SOLOIL Incorporated (a Romualdez company) by Ricardo Lopa, President Aquino's brother-in-law, and called for a Senate investigation into possible violations of R.A. No. 3019 (Anti-Graft and Corrupt Practices Act).
• The matter was referred to the Senate Blue Ribbon Committee, which began an investigation into the "sale of the thirty-six (36) corporations belonging to Benjamin 'Kokoy' Romualdez."
• Petitioners and Ricardo Lopa were subpoenaed. At a hearing on May 23, 1989, Lopa declined to testify, fearing his testimony might prejudice defendants in Civil Case No. 0035. Petitioner Jose F.S. Bengzon Jr. also refused, citing his right to due process and potential adverse effects on his rights as a co-defendant in the Sandiganbayan case due to publicity.
• The Committee suspended its inquiry, asked for memoranda on constitutional issues, and then issued a resolution on June 5, 1989, rejecting petitioners' plea and deciding to continue the investigation.
• Petitioners filed this petition for prohibition, arguing the Committee's actions were in excess of jurisdiction, lacked legislative purpose, violated their constitutional rights, and would cause irreparable damage.
• Jose S. Sandejas, another defendant in Civil Case No. 0035, was allowed to intervene in this Supreme Court case.

• The Senate Blue Ribbon Committee's inquiry has no valid legislative purpose and is not done in aid of legislation.
• The sale or disposition of the Romualdez corporations is a "purely private transaction" beyond the Committee's power to inquire into.
• The inquiry violates their constitutional right to due process, as the publicity generated could adversely affect their rights as defendants in the pending Sandiganbayan Civil Case No. 0035.
• Compelling them to testify would prejudice their defense in the Sandiganbayan case, which involves the same subject matter as the Senate inquiry.
• The Committee's investigation constitutes an encroachment on the jurisdiction of the Sandiganbayan.

• The Supreme Court cannot inquire into the motives of lawmakers in conducting legislative investigations under the doctrine of separation of powers.
• The inquiry is being conducted in aid of legislation, pursuant to Senator Enrile's privilege speech and Senate Resolution No. 212 (which concerned PCGG activities).
• The power of Congress to conduct inquiries in aid of legislation is broad and necessary for the legislative process.
• The petitioners, as witnesses, can only invoke the right against self-incrimination on a question-by-question basis and cannot refuse to testify altogether.

• Whether the Supreme Court has jurisdiction to inquire into the scope and extent of the Senate Blue Ribbon Committee's power to conduct inquiries into private affairs in purported aid of legislation.
• Whether the Senate Blue Ribbon Committee's inquiry into the alleged sale of Romualdez corporations to the Lopa Group is in aid of legislation.
• Whether the said inquiry violates the petitioners' constitutional rights, specifically the right to due process and the right against self-incrimination, considering the pending Sandiganbayan case (Civil Case No. 0035) involving the same subject matter.
• Whether the Senate inquiry constitutes an encroachment upon the judicial powers of the Sandiganbayan.

• The petition is GRANTED. The Supreme Court held that it has jurisdiction to determine the scope and extent of the power of the Senate Blue Ribbon Committee to conduct inquiries.
• The Court found that the contemplated inquiry by the respondent Committee was not "in aid of legislation." Senator Enrile's speech, which triggered the inquiry, aimed to determine if relatives of President Aquino (specifically Ricardo Lopa) had violated R.A. No. 3019, a matter more within the province of courts than the legislature. Senate Resolution No. 212, cited by respondents, pertained to an investigation of the PCGG and was not the basis for the inquiry into the Lopa transaction.
• The Court ruled that allowing the Committee to investigate an issue already before the Sandiganbayan (pre-emption) would pose a risk of conflicting judgments and potential influence on the Sandiganbayan's decision, thus encroaching upon the exclusive domain of judicial jurisdiction.
• Given that the petitioners are defendants in a pending Sandiganbayan case involving issues intimately related to the subject of the Committee's inquiry, and the inquiry is not in aid of legislation, compelling them to testify would violate the principle of separation of powers and their rights. The Court distinguished this from merely invoking the right against self-incrimination per question, stating that petitioners cannot be compelled to appear and testify at all because the inquiry itself is improper.

• Separation of Powers — A fundamental principle where governmental powers are divided among the executive, legislative, and judicial branches, each supreme within its own sphere. The Court applied this by stating that while it generally doesn't inquire into legislative motives, it has the duty to determine proper allocation of powers and prevent one branch from encroaching on another. Here, the Senate inquiry was found to encroach on the judicial power already exercised by the Sandiganbayan over the same matter.
• Power of Legislative Inquiry (In Aid of Legislation) — The power of Congress to conduct inquiries is recognized by the Constitution (Art. VI, Sec. 21) but is not absolute. It must be "in aid of legislation," conducted in accordance with duly published rules of procedure, and must respect the rights of persons appearing. The Court found the inquiry lacked a legitimate legislative purpose as it was aimed at investigating an alleged crime (violation of R.A. 3019) rather than gathering facts for potential legislation.
• Right Against Self-Incrimination (Art. III, Sec. 17, Constitution) — No person shall be compelled to be a witness against himself. The Court reiterated that an accused in a criminal case can refuse to take the stand altogether, while an ordinary witness can only invoke the right as incriminating questions are asked. However, in this case, because the legislative inquiry itself was deemed improper (not in aid of legislation and encroaching on judicial power), the petitioners (who were defendants in a related Sandiganbayan case) could not be compelled to testify or produce evidence before the Committee at all.
• Judicial Review (under Art. VIII, Sec. 1, Constitution) — The duty of the courts to settle actual controversies involving legally demandable and enforceable rights and to determine grave abuse of discretion by any branch of government. The Court invoked this to assert its jurisdiction to delimit constitutional boundaries and review the Committee's exercise of its inquiry power.
• Sub Judice / Pre-emption by the Judiciary — While not explicitly termed "sub judice rule" in the main ruling, the Court emphasized that for the Senate Committee to probe into the same justiciable controversy already before the Sandiganbayan would be an encroachment into the exclusive domain of judicial jurisdiction. This pre-emption by the court over matters already sub judice was a key reason for granting the prohibition.

• "The power of congress to conduct investigations is inherent in the legislative process... But broad as is this power of inquiry, it is not unlimited. There is no general authority to expose the private affairs of individuals without justification in terms of the functions of congress... Nor is the Congress a law enforcement or trial agency. These are functions of the executive and judicial departments of government. No inquiry is an end in itself; it must be related to and in furtherance of a legitimate task of Congress." (citing John T. Watkins vs. United States)
• "To allow the respondent Committee to conduct its own investigation of an issue already before the Sandiganbayan would not only pose the possibility of conflicting judgments between a legislative committee and a judicial tribunal, but if the Committee's judgment were to be reached before that of the Sandiganbayan, the possibility of its influence being made to bear on the ultimate judgment of the Sandiganbayan can not be discounted."
• "If we presently rule that petitioners may not be compelled by the respondent Committee to appear, testify and produce evidence before it, it is only because we hold that the questioned inquiry is not in aid of legislation and, if pursued, would be violative of the principle of separation of powers between the legislative and the judicial departments of government, ordained by the Constitution."

• Angara vs. Electoral Commission — Cited to affirm the Supreme Court's power of judicial review to determine the proper allocation of powers between government departments and to delimit constitutional boundaries.
• Jean L. Arnault vs. Leon Nazareno, et al. — Referenced for the principle that a legislative inquiry must be material or necessary to the exercise of a power vested in it by the Constitution, such as to legislate.
• John T. Watkins vs. United States — Extensively quoted to delineate the limits of congressional investigative power, emphasizing it must be for a legislative purpose and not to expose private affairs or act as a law enforcement/trial agency.
• Baremblatt vs. United States — Cited to support the limitation that Congress cannot inquire into matters within the exclusive province of other branches of government, particularly the Judiciary.
• Romeo Chavez vs. The Honorable Court of Appeals, et al. — Referenced for the distinction in the application of the right against self-incrimination: an accused may refuse to take the stand entirely, while an ordinary witness may only invoke it per incriminating question.
• Galman vs. Pamaran (citing Cabal vs. Kapunan) — Cited to illustrate that the privilege against self-incrimination extends to all proceedings sanctioned by law where punishment is sought, regardless of the character of the suit.

• Article VI, Section 21, 1987 Constitution — This provision grants the Senate or House, or their committees, the power to conduct inquiries in aid of legislation, in accordance with published rules, and respecting the rights of persons affected. The Court found the Committee's inquiry did not meet the "in aid of legislation" requirement.
• Article VIII, Section 1, 1987 Constitution — This section defines Judicial Power, including the duty to settle actual controversies and determine grave abuse of discretion. It was the basis for the Court's jurisdiction to review the Committee's actions.
• Article III, Section 17, 1987 Constitution — This is the self-incrimination clause ("No person shall be compelled to be a witness against himself."). The Court held that compelling petitioners to testify in an improper inquiry related to a pending case would violate their rights.
• Republic Act No. 3019 (Anti-Graft and Corrupt Practices Act), Section 5 — Senator Enrile's speech, which initiated the Senate inquiry, called for an investigation into a possible violation of this section (Prohibition on certain relatives). The Court noted this indicated the inquiry's purpose was prosecutorial rather than legislative.
• Senate Rules of Procedure Governing Inquiries in Aid of Legislation (Sec. 1 and Sec. 4) — Mentioned as the framework under which Senate inquiries are supposed to operate. Section 1 defines the scope of inquiries, and Section 4 allows referral of speeches for inquiry if deemed appropriate for aid in legislation.

• Justice Cruz — Argued that the investigation was in aid of legislation, presuming a legitimate legislative object. He stated that the legislature has a right to investigate public fund disposition and possible violations of law for amendatory legislation. He believed petitioners could invoke the right against self-incrimination per question but not refuse to testify altogether, as they were not accused before the Committee. He cautioned against unduly trenching on legislative prerogatives absent a clear grave abuse of discretion.
• Justice Gutierrez, Jr. — Expressed a strong dissent, asserting the Court was encroaching on Congress's constitutionally vested power to conduct investigations in aid of legislation and second-guessing legislative motives. He argued that the power of inquiry is inherent and broad, and unraveling the Romualdez-Lopa transactions could result in useful legislation. Citing US jurisprudence (McGrain v. Daugherty), he emphasized the presumption of legitimate legislative purpose and that an express avowal of such purpose is not indispensable. He maintained that a pending judicial case does not halt legislative investigations, as their purposes are distinct, and petitioners' rights are protected by invoking self-incrimination for specific questions.