The emphasis in the social-psychological collective action literature is on why individuals take part in collective action; however, it does not elaborate on how different mobilizing contexts may appeal to distinct motivational dynamics to participate. The present study connects the microlevel of motivational dynamics of individual protesters with the mesolevel of social movement characteristics. To do so a field study was conducted. Protesters were surveyed in the act of protesting in two different demonstrations in two different town squares simultaneously organized by two social movements at exactly the same time against the same budget cuts proposed by the same government. But with one fundamental difference, the movements emphasized different aspects of the policies proposed by the government. This most similar systems design created a unique natural experiment, which enabled the authors to examine whether the motivational dynamics of individual protesters are moderated by the social movement context. Previous research suggested an instrumental path to collective action, and the authors added an ideology path. The authors expected and found that power-oriented collective action appeals to instrumental motives and efficacy and that value-oriented collective action appeals to ideological motives, and, finally, that efficacy mediates on instrumental motives and motivational strength, but only so in power-oriented action. © 2009 The Society for the Psychological Study of Social Issues.