Alliances are increasingly considered a key element for innovation, especially in knowledge‐intensive firms. While this is true, the mere membership to alliances does not explain innovation performance, and thus the alliance's characteristics that determine high performance must be examined. This research address the question of how the diversity of partners in a certain alliance for innovation affects innovation performance, and how this influence can be moderated by certain characteristics, such as the relational dimension of social capital and the type of knowledge shared among partners. The empirical analysis of a sample of 90 biotech companies shows that there is an inverted U‐shaped relationship between alliance partner diversity and innovation performance and confirms the positive moderating effects of relational social capital and knowledge codifiability. These findings contribute to the current research on alliances for innovation by providing empirical evidence on why some alliances perform better than others. Also, the results suggest that the study of alliance partner diversity, as a determinant of alliance performance, should not be addressed in isolation.