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 collective-identity possession was undermined when an identity symbol was available that could function as a surrogate for more costly identity-affirming behaviour. Further conceptual and social implications of the identity-affirming function of social movement support are discussed. Copyright © 2007 John Wiley & Sons, Ltd.