This dissertation explains the diversity of multilateral paths followed by similar small rentier states in the international realm. Why do some states commit to multilateralism by signing legally binding treaties, participating in institutions and contributing financial resources? Amongst small state theorists, common wisdom has long held that small states are more likely to act multilaterally because of structural needs to bandwagon with bigger actors within existing frameworks. Liberal scholars reach a similar conclusion by arguing that states are progressively “socialized” within international organizations. On the other hand, political economists indicate that when a rentier state enjoys preferential trading terms thanks to its hydrocarbons exports, it has little incentive to engage multilaterally. “Branding” scholars in particular consider that small rentier states have a preference for costly and visible initiatives and little interest for the nitty-gritty aspects of international cooperation. The predictions outlined by these two research strands are completely antagonistic: while small state theory and liberalism predict more multilateral cooperation, rentier state theory predicts less of it. These opposite views fail however to explain the diversity of approaches to multilateral cooperation of small, resource-rich states like the Gulf emirates of Kuwait, Qatar and the United Arab Emirates (UAE) – but also Norway, Venezuela or Brunei, for instance. Against  this  background,  this  dissertation  endeavours  to  understand  better  this  discrepancy with a case study approach of two similar oil and gas exporting small states, Qatar and the UAE. These follow significantly different multilateral approaches in the field of foreign aid, a key component of their foreign policy for which they are increasingly important globally. The two city states diverge on two major items of the international community’s aid agenda – donor proliferation and aid transparency. On the one hand, the UAE has in the past decade opened its doors to most Western international organizations of foreign assistance and become an active member of the OECD Development Assistance Committee on sensitive issues such as aid transparency; and made exceptional efforts in the region and among emerging donors at large, to streamline its aid landscape. On the other hand, Qatar has favoured bilateral frameworks and shut out the United Nations Development Program in Doha. Its attempt to streamline aid, despite reforms initiated fifteen years ago, has not as yet been fruitful.