Many researchers and business professionals have emphasized the importance and difficulties of successful information systems (IS) integration in the context of mergers and acquisitions (M&A). However, existing research remains sparse, failing to explain how firms design their IS integration strategy and its relation to successful IS integration. In order to overcome this shortcoming, we adapt six dimensions of strategic IS planning to the post-merger integration situation, namely comprehensiveness, formalization, focus, flow, participation, and consistency in the post-merger IS integration context. We then use two indepth cases studies to shed light on these constructs. We find evidence that contradicts the typical view presented in the M&A literature, and argue that there are several different approaches to post-merger IS integration planning. In the analysis, we point out specific differences between the cases that eventually lead up to these fundamentally different approaches to IS integration design.