Civil society organizations choose vastly different forms of collective action to try to influence European politics: everything from insider lobbying to disruptive protest, from public education to hunger strikes. Using network analysis and qualitative interviewing, my research emphasizes that patterns of inter-organizational relations influence organizational decisions to use one of these strategies. They do this by structuring the information and resources available to actors, as well as by diffusing strategies across connected actors. This is particularly true when networks are segmented into two distinct components, as I find in the European climate change network. In this network, organizations using contentious 'outsider' strategies are only loosely linked to those 'insiders' behaving conventionally in Brussels. These findings are policy relevant because current scholarship and policy recommendations tend to assume that increased civil society participation in transnational policy-making will increase democratic legitimacy. But my network data and qualitative interviews suggests that the emergence of a coalition of organizations engaging solely in contentious outsider action reflects the development and diffusion of a new and highly critical strand of climate change politics. I further argue that this type of contentious civil society 'spillover' can actually slow the pace of development of climate change policy and of European integration more generally.