In this paper we aim to improve our understanding of the dynamics of IS architecture complexity (i.e, the change in this complexity over time) during the execution of a consolidation IS integration strategy (IIS). Based on two case studies, we find that unexpected levels of complexity emerge during IIS execution because of an underestimation of requisite complexity and an overestimation of the potential to reduce complexity. Our analysis shows that increased complexity is due to the fact that the intended consolidation IIS is only partially executed, and to increasingly emergent IIS execution. Additionally, we find that while complexity was reduced at the portfolio level, at more detailed levels of observation complexity was actually increased. Our paper contributes to knowledge in the field by providing a deeper insight into IS architecture complexity dynamics during the execution of a consolidation IIS, and the concept of IS architecture complexity in general.