We propose and test a framework of the antecedents of contribution in two technology-mediated citizen science projects, with different degrees of task granularity. Comparing earlier findings on the motivations of volunteers in a web-based image analysis project (high granularity), with new findings on the motivations of volunteers in a volunteer computing project (low granularity), we found that participation task granularity is correlated with motivation levels. Further, we found that collective and intrinsic motives are the most salient motivational factors, whereas reward motives are less important for volunteers. Intrinsic, norm-oriented and reputation-seeking motives were most strongly associated with participation intentions, which were, in turn, associated with participation. Finally, comparing the relationship between motives and participation among the two volunteer populations, we found that active-participation volunteers are characterized by significantly stronger association between collective motives and contribution intention, whereas passive-participation volunteers are characterized by significantly stronger association between identification with the community and contribution intention. Implications for research and practice are discussed.