This article provides a new mechanism in understanding how partner heterogeneity moderates an alliance’s ability to advance corporate social responsibility goals. I identified the antecedents for firms to select a more diverse set of partners and explored whether more diverse alliances (especially cross-sector alliances) may facilitate partners to achieve more proactive environmental outcomes. I employ 146 environmental alliances formed in the U.S. between 1990 and 2009 to test the assertions. Results suggest that firms with innovative orientation and alliance experiences tend to choose a more diverse set of partners (especially cross-sector partners); and such partner heterogeneity in turn moderates an alliance’s environmental outcomes—compared to inter-firm alliances, cross-sector alliances are more likely to facilitate partners to pursue more proactive environmental strategies.