In this paper, regime complexes are conceptualized as dynamic networks constituted by relations between international organizations (IOs). We introduce "IO positioning"as a conceptual lens for studying patterns and shifts in IO networks resulting from negotiations between IOs over their distinctiveness and social membership in complex organizational fields. We suggest that IO positioning has two constitutive effects. First, on the level of individual IOs, positioning affects IO identities within the field as these are (re)negotiated in relations with other organizations. Secondly, the positioning practices of IOs have constitutive effects on the contours of entire policy fields too; they form and shift the boundaries of regime complexes. Empirically, the paper examines the utility of our approach by analyzing the history, dynamics, and positioning effects of interorganizational relations between eight IOs in global health governance - an area of international cooperation that is commonly portrayed as exceptionally fragmented, complex, and densely populated. Examining relations between our eight IOs, we provide network analytical longitudinal data of in- and out-reporting by IOs derived from IOs' annual reports between 1970 and 2017. We triangulate our network analysis with data derived from semi-structured interviews with health IO professionals.