Organizations constantly adapt their Information Systems (IS) architecture to reflect changes in their environment. In general, such adaptations steadily increase the complexity of their IS architecture, thereby negatively impacting IS efficiency and IS flexibility. Based on a Complex Adaptive Systems (CAS) perspective, we present a more differentiated analysis of the impact of IS architecture complexity. We hypothesize the relation between IS architecture complexity on the one hand, and IS efficiency and IS flexibility on the other hand to be mediated by evolutionary and revolutionary IS change. Subsequently, we test our hypotheses through a partial least squares (PLS) approach to structural equation modelling (SEM) based on survey data from 185 respondents. We find that the direct negative impact of IS architecture complexity on IS efficiency and IS flexibility is no longer statistically relevant when also considering the mediating effects of revolutionary and evolutionary IS change.