Recent research suggests that community-based collaboration may build social capital-defined as trust, norms of reciprocity, and networks. Social capital may improve a group's ability to collaborate, manage risk, innovate, and adapt to change. We used mail surveys of group participants and key informant interviews to assess whether the following collaborative group characteristics affected social capital built within 10 collaborative groups in northwest Colorado: perceived success, conflict, activeness, stakeholder diversity, previous collaboration experience, similar values and beliefs, group size, group age, and initial social capital. Perceived success and initial levels of social capital were the strongest predictors of current levels of and changes in social capital over time. Collaboration experience negatively influenced current levels of trust. Our results suggest that collaborative groups may need to consider the outcomes of collaborative interactions in order to build social capital.