The involvement of all relevant stakeholders into the elaboration process of shared goals is generally considered as a key factor for a successful regional development. However, no empirical research has been conducted to confirm this claim yet. In the last years, increasing efforts have been made to evaluate participatory processes, but the focus of that research was on the quality of the processes. This paper presents a recent study which had the objective to measure the effect of consensus building processes, a particularly interactive form of participation, in a systematic and reliable way. To this end, a new evaluation method based on the methodology of intervention research of environmental psychology was developed and tested in a case study.