It is argued that the power of collective identification to mobilize people for collective action such as social movement support derives at least partly from processes of identity affirmation. The hypothesized identity-affirming function of social movement support is tested in two laboratory experiments which revolve around collective identity as a supporter of the peace movement. In Experiment 1, we predicted and found that people who strongly identified with the peace movement showed more movement support (i.e. made more monetary donations to the peace movement) under conditions of uncertain as opposed to certain possession of identity as a movement supporter. In Experiment 2, we replicated this finding, but also found, in accordance with the notion of substitution, that the mobilizing effect of uncertain collectiveidentity possession was undermined when an identity symbol was available that could function as a surrogate for more costly identity-affirming behaviour. 'The ''good'' thing about the Iraq war is that now our peace movement will be strengthened again. ' (anonymous supporter of the peace movement, March 2003)In recent years social psychology has made important contributions to a better understanding of collective phenomena that had long been considered the traditional domain of sociological theorizing and research. More specifically, guided by the social identity or self-categorization perspective (Tajfel & Turner, 1986;Turner, Hogg, Oakes, Reicher, & Wetherell, 1987), social psychological research demonstrated that collective identification is an important determinant of social movement support. From this perspective, social movement support is the enactment of a particular politicized group or social category membership, and collective identification operates as the basic social psychological process underlying movement support (Simon & Klandermans, 2001). Accordingly, social psychological research has uncovered the facilitative role of collective identification in mobilization for political action in various social movement contexts, including the women's movement, the older people's movement, the gay movement, the fat acceptance movement, the farmers' movement and right-wing political movements (e.g. De Weerd & Klandermans, 1999;Kelly & Breinlinger, 1996;Simon et al., 1998; for reviews, see Simon, 2004;Stürmer & Simon, 2004;Wright, 2001).Although the motivation to support a social movement is rarely free from cost-benefit calculations revolving around more tangible interests of the individual and his or her group (Klandermans, 1997), recent research also indicates that collective identification possesses a unique mobilizing power over and above such calculative considerations (Simon, 2004;Stürmer & Simon, 2004). In a first attempt to uncover the psychological process underlying the mobilizing power of collective identification, Stürmer, Simon, Loewy and Jörger (2003) predicted and found that a sense of inner obligation to