The introduction of Enterprise Identity Management Systems (EIdMS) in organisations is a costly and challenging endeavour, beyond the technological level. Especially for decision makers it seems difficult to fully understand the impacts and potentials from introducing EIdMS, such as the relevant aspects, for an ex-ante evaluation. The qualitative expert interview study presented in this paper was conducted to better understand the “hybrid” nature of EIdMS, being situated between security and productive IT systems. To this regard, the focus is put on the general nature of EIdMS projects and categories being relevant for analysing such. Based on the derived categories and insights from the interviews, a theoretical model for the evaluation of EIdMS introductions is developed, which can be used to derive adequate decision support systems.