Effectively managing issues of environmental risk requires learning and collaboration within policy networks. Within the environmental policy process, networks of information sharing, resource exchange, and other forms of interaction allow organizations to synthesize information and work towards shared goals. Ultimately, this allows policy actors to collectively learn how to deal with complex, uncertain, and emerging challenges. Despite the importance of policy networks, however, the forces that shape these structures—and the possible institutions that can promote more effective networks—are not well understood.  According to the Advocacy Coalition Framework, the dynamics of policy network formation lead to structures exhibiting belief-oriented segregation—that is, a high correspondence between shared policy beliefs and voluntary collaborative relationships. These structures may be produced through at least two pathways: belief homophily, where actors actively seek out connections with others sharing their belief system, and learning, where policy beliefs diffuse through collaborative ties. The cross-sectional design of many policy network studies precludes an explicit examination of these potentially complementary forces. This paper explicitly examines these dynamics using a reanalysis of data on policy beliefs and networking in U.S. environmental risk policy across two time periods, 1984 and 2000 (N = 223). Results indicate strong homophily effects, but relatively weak learning effects, in the evolution of this policy network. This research helps pave the way for additional research on the dynamics that share policy networks and beliefs, and also helps to clarify the differences between individual versus organizational contributions to policy network evolution