Adequately considering interactions among IT/IS projects in the process of constructing an IT/IS project portfolio is an important requirement for IT/IS project portfolio selection. Especially interactions caused by the simultaneous utilization of scarce resources are very common and of high relevance for IT/IS project portfolio selection problems. In the literature, resource interactions are already incorporated into technically sophisticated optimization models, but it is assumed that the necessary input data are entirely available to the planner. The identification and assessment of resource interactions are not supported sufficiently and thus, the applicability of these optimization models in business practice is severely aggravated. In this article we extend the prevailing definitions of resource interactions in the literature by introducing allocation, performance, and sourcing interactions. Using these three types of interactions, we present a structured identification process for resource interactions among IT/IS projects, which is the main contribution of this article. Further, we present a tailored mathematical decision model that accounts for the identified interactions.