This article attempts to discover why states have chosen to co-ordinate within ad hoc informal international organizations instead or on top of formal ones, in order to resolve the Iranian nuclear crisis in the period 2003-15. Three informal groups of statesthe E3 (France, Germany and the UK), the EU-3 (the E3 + the EU High Representative), and the EU-3 + 3 (the EU-3 + the US, Russia, and China)are widely seen as having contributed to the diplomatic solution. Empirically, one can wonder why formal international organizations like the UN, the IAEA and the EU were not able to do the wheeling and dealing by themselves in the case of the Iranian nuclear conflict. From a theoretical perspective, this article highlights the role of informal international organizations, a subject that is under-researched both in EU and International Relations studies. It will argue that these temporary informal international organizations have three advantages: no (or less) bureaucracy, speed, and the ability to buy time for more important diplomatic actors like major states or formal international organizations.