This paper explores the role of ideological division in complicating collaboration between third sector organizations working to influence European Union (EU) policy processes. The Social Platform, the largest alliance of social justice groups working at EU level, is studied to illustrate how a coalition of ideologically disparate organizations cope with internal tensions that are often exacerbated by external shifts in the political opportunities available for mobilization. The alliance is successful at mediating the most significant sources of ideologically based division. However, the external political context for mobilization requires coalition members to compete for recognition and resources available at EU level. The reordering of EU equality legislation and initiatives has most recently exacerbated a series of competitive dynamics in play across the EU third sector and has stretched the ability of the coalition to maintain unity among member organizations. Â© 2010 Taylor & Francis.