Scholars of international relations have devoted significant effort to understanding international organisations. However, two areas have been understudied: the role of the chair in international multilateral negotiations and the role of informal international organisations. Yet informal international organisations are increasingly important in international affairs as world leaders turn to smaller and more flexible forums to address global challenges. This article addresses these two blind spots in the literature by considering the role of Australia as chair in one of the most important yet most understudied informal international organisations: the Group of Twenty (G20). Drawing on primary interview data and the participant observations of the first author, who was a member of the G20 chair in 2013-14 during Australia's presidency, the authors examine two theoretical puzzles: (1) why states delegate control of the negotiation process to a chair and (2) how the chair can, and does, influence the negotiation process. It is argued that member states delegate control to the chair to overcome specific institutional failures and, in doing so, provide the chair with the power to influence the negotiation process. The authors also argue that the G20 case indicates that existing theory overlooks key factors which restrict the capacity of the chair to influence the negotiation outcome.