The objective of this research in progress paper is to develop a theory-driven model to categorise post-implementation modifications to ERP systems and to understand the relationship between the identified categories and business process outcomes that are generated as a result of the modifications. While ERP systems can improve the efficiency, effectiveness and flexibility of business processes, the relationship between these outcomes and post-implementation modifications is not understood adequately. The model proposed here provides a theoretical foundation for research into the impact of modifications on business process performance, and brings clarity to the definition of an ERP modification by developing a typology of modifications.