The veto power of the permanent members of the United Nations (UN) Security Council has, since the establishment of the UN, consistently been one of the most controversial mechanisms of the collective security system, particularly in situations where circumstances arising from serious breaches of peremptory norms come before the Council. Focusing on this issue, the present article examines the legitimacy of the use of the veto in such situations. First, drawing on general international law, international judicial practice, and the internal logic of the UN Charter, the article demonstrates that the Council is bound—like other subjects of international law—by peremptory norms and cannot regard itself as operating beyond them. It then refers to the ILC’s Draft Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA) and Draft Articles on Responsibility of International Organizations (DARIO) to analyze the specific consequences of serious breaches of peremptory norms, particularly the obligation to cooperate to bring such breaches to an end and the obligation of non‑recognition of situations resulting from them. The findings of the article indicate that the use of the veto, where it results in the inaction of the Council in the face of such situations, conflicts with the obligation to cooperate in bringing serious breaches of peremptory norms to an end. Nevertheless, the veto does not, in itself, amount to recognition of an unlawful situation and therefore does not necessarily constitute a breach of the obligation of non‑recognition.