Previous experimental and empirical evidence has identified social preferences in the voluntary provision of public goods. A number of competing models of such preferences have been proposed. We provide evidence for one model of behavior in these games, reciprocity (or matching, or conditional cooperation). Consistent with previous research, we find that participants in the voluntary contribution mechanism attempt to match the contributions of others in their group. We also examine participants in a related game with different equilibria, the weakest-link mechanism. Here, in contrast, participants contribute so as to match the minimum contribution of others in their group.