This article reports on the evaluability assessment (EA) of a program to prevent substance use among young people transitioning from primary to secondary school. Termed Network Intervention (NI), the program is implemented among fifth- and sixth-grade students from five primary schools within a single school board in Quebec. Drawing from qualitative data gathered via interviews, meeting notes and the patient navigator’s logbook, the study allowed a better understanding of the program and its logic model, a tool necessary for the coming evaluation activities. The EA has proven useful not only for the evaluation team but also for the initiators of the program and those involved in the implementation. Recommendations are made to better conduct the process of an EA.