Building on organizational learning theory, this study examines whether firms learn to manage alliance portfolio diversity. We argue that alliance portfolio diversity may be advantageous as well as disadvantageous for alliance portfolio performance. Subsequently, we theorize that the diversity-performance relationship is moderated by the firm's alliance experience and capability. Bridging the previously separated literature of diversity and learning, our findings show a curvilinear relationship between diversity and performance. More important, using survey data, we reveal some key processes through which firms learn to manage alliance portfolio diversity.