How do firms create strategically relevant capabilities? In this study, insights from evolutionary economics and organizational learning theories are combined to explore the mechanisms behind the creation of organizational capabilities in the context of infrequent, heterogeneous, and complex administrative tasks. More specifically, the investigation covers the effects of both tacit and codified knowledge accumulation mechanisms on the development of a practice specialized in the management of post-acquisition integration processes. Hypotheses about the performance implications of pre-acquisition resources, post-acquisition integration decisions, and knowledge accumulation and codification processes are tested with primary data collected from a sample of 51 bank holding companies in the United States and Canada, for a total of 577 completed acquisitions. Results show that codification and routinization processes play a key role in shaping the evolution of post-acquisition integration practices, and that both mechanisms have a positive influence on acquisition performance, within specific limitations. The effectiveness of tacit knowledge accumulation is constrained by the degree of homogeneity of past experiences, whereas knowledge codification impacts performance only when high levels of integration are to be achieved. Results also show that greater level of integration have positive implications for acquisition performance and that decisions to replace top management affect performance negatively. Conclusions are drawn about necessary refinements of current theoretical approaches to accommodate complex learning conditions, and the potential implications for the management of acquisitions, as well as other infrequent and complex organizational events, such as strategic alliances and internal reorganizations, are discussed.