The study comprises an analysis of processes of psychological change among participants at an environmental protest. A participant observation study found evidence of a radicalized self concept among a number of crowd members, and indicates a link between radicalization, an asymmetry of categorical representations between protesters and the police, and the subsequent interaction premised on these divergent representations. The analysis supports an elaborated social identity model of crowd behaviour (Reicher, 1996, 1997a, 1997b; Stott & Reicher, 1998). It is argued that, in order to account for both social determination and social change in collective behaviour, it is necessary to analyse crowd events as developing interactions between groups. Where crowd members hold a different understanding of their social position to that held by an out-group (e.g. the police) and where the out-group has the power to treat crowd members in terms of its understandings, then those members who act on the basis of one understanding of their social relations find themselves in an unexpected and novel set of social relations. This then provides the basis for a series of changes, including the self-understanding of crowd members.