Since the end of WWII, the numbers of international organizations (IOs) and regional organizations (ROs) have increased tremendously. Today there are hardly any policy areas in which states do not cooperate in IOs and hardly any geographical regions without ROs. This paper sheds light on one particular linkage between ROs and IOs. It measures and analyzes the activity of more than 20 of the most prominent ROs in more than 200 negotiations (between 2008 and 2012) within the United Nations system (UNGA, UNCTAD, UNFCCC, UNESCO, HRC, ECOSOC, UNEP, UNHCR, ILO, WHO, WIPO, IAEA). The paper illustrates that the activity of ROs in negotiations varies considerably between both ROs and IOs and presents a theoretical multilevel framework to explain the observed pattern. A quantitative hypotheses test shows that the combination of IO, RO and member states properties account for variation in RO activity across UN negotiations.