This research builds on the complementary corporate social responsibility (CSR) literatures in strategy and marketing to provide insight into the efficacy of CSR as a challenger's competitive weapon against a market leader. Through an investigation of a real-world CSR initiative, we show that the challenger can reap superior business returns (i.e., more positive attitudinal and behavioral outcomes) among consumers who had participated in its CSR initiative, relative to those who were merely aware of the initiative. Specifically, participant consumers demonstrate the desired attitudinal and behavioral changes in favor of the challenger, regardless of their affective trust in the leader, whereas aware consumers' reactions become less favorable as their affective trust in the leader increases. Furthermore, participant consumers, but not aware ones, form a communal, trust-based bond with the challenger. This paper was accepted by Pradeep Chintagunta and Preyas Desai, special issue editors. This paper was accepted by Pradeep Chintagunta and Preyas Desai, special issue editors.