Since the end of the Second World War, the power of international organisations has expanded dramatically, throwing the frequently negative impact of their activities on human rights and the rule of law in to sharp relief. The topic of the responsibility (and accountability) of international organisations and their member states has thus received considerable attention in legal scholarship. There is, however, one aspect of the problem which remains largely neglected, namely the individual responsibility of states for their participation in the decisionmaking of the international organisations to which they belong. This is because, traditionally, legal scholars have denied such responsibility on the basis of the separate legal personality of international organisations, which—akin to a “corporate veil”—conceals member states actions in the institutional settings from any form of legal scrutiny. Contrary to this traditional view, this thesis aims—in the first instance—to bring attention to the previously neglected practice of endowing member states with legal responsibility for their own conduct in the decision-making processes in international organisations. Specifically, member states’ legal responsibility for their voting behavior or—in cases of decision-making by consensus—the specific position assumed. The second aim of the study is to construct a more nuanced conceptual framework, elaborating how such member state responsibility should be understood beyond the conventional International Law Commission responsibility regime. Building on both international institutional law and legal theory, this thesis employs (a modified version) of Kelsen’s theory of sovereignty as a juridical concept to ultimately argue that member state responsibility in decision-making processes should be understood as a due diligence obligation to ensure the unity of law. In other words, it is argued that member states have a duty to take into account the multiplicity of obligations under both international law and the legal order of the organisation to which they belong. At a more practical level, this boils down to the duty of legal reasoning when voting and deliberating in international organisations.