Mergers and acquisitions (M&A) are frequently used as an approach for organisational growth and expansion. One of the crucial phases of M&A is post-merger integration (PMI) when practical integration of selected organisations is implemented. This phase is intended to create a new future organisation, which will be capable to achieve the goals selected for M&A. For this, PMI should implement aligned changes in all enterprise architecture levels from business to information system (IS). However, in practice, changes on business level and IS level are frequently misaligned, which leads to IS integration related decisions made with no sufficient knowledge of M&A context and requirements for future IS architecture. This research explores how existing requirements engineering (RE), enterprise architecture (EA), and knowledge management (KM) disciplines can contribute to PMI IS integration process. PMI IS integration should be based on the comparison of IS architectures in merging organisations and focused on the required atomic decisions about consolidation of IS with similar goals. It should consider the M&A context and be applicable in the PMI context with high time pressure and limited explicit knowledge.