Abstract:Whether we look at constitutions of states or founding treaties of International Organisations (IO), it is striking that many rules on interaction between delegates create room for deliberation, whilst simultaneously limiting the time for discussion. While the latter speeds up decision making, it risks reducing its quality and legitimacy by hampering the exchange and contestation of information and ideas. How are these competing elements balanced in IOs? Do IOs differ in this respect, and if so, how and why? The article draws on a unique and novel dataset and assesses variation in the extent to which institutional design fosters or inhibits diplomatic deliberation in more than 110 diverse IOs. To this end, the article uses a combination of theories of functionalism, rational choice institutionalism and liberal approaches on variation, fit, and mismatch of deliberative institutional design within and across IOs. The hypotheses are analysed with quantitative methods. The article shows that diplomatic deliberative institutional design elements are the most pronounced when IOs are small in size, deal in high politics, and are regional in character.