One of the important activities after the decision to merge organizations is made is the postmerger integration (PMI) of organizations, including the integration of their information systems (IS). This research focuses on the decisions and plan phase in the post-merger integration and aims to answer the following question - how decisions on PMI IS integration should be organized for better M&A results? In this research, merger and acquisition (M&A) context and PMI are integration-related literature is reviewed to determine factors that can impact PMI IS integration decision-making and correspondingly M&A results. Based on findings, the PMI IS integration decision requirements model is created and applied to the previously created PMI-specific requirements engineering, knowledge management, and enterprise architecture-based framework, uncovering required adjustments to the previously defined PMI IS decision-making process.