Research impact is high on the agendas of governments and funders all over the world. As participatory action-oriented research (PAR) intends to have an impact – positive changes in policy and practice – interest in PAR is rising. But although impact may sound clear, it is susceptible to various interpretations. Hence assessing the impact of PAR is a challenge. In this article we describe a PAR project with multi-stakeholder participation at its core, and focus on the effectiveness (co-impact) of this project. We elaborate on two interpretations of impact: findings-based and process-based impact. Although interest in research impact tends to prioritise findings-based impact which focuses on substantive results and shows little or no attention for the effects of the participation process (process-based impact), we acknowledge both types of impact. We equally value the process-based impact, as it does justice to the potential richness of participation. In presenting our research approach and findings, we provide insight in how both findings-based (substantive results) and process-based (less tangible effects) impact can be generated and assessed. We experienced that strict vigilance was crucial for detecting process related effects, without comprising the findings-based ones. Our research approach, with its consciously chosen sequence of actions, a variety of data gathering methods, and informed selection of stakeholder groups, appears to pay off in generating co-impact. Additionally, the operationalisation of a conceptual impact framework in an applicable impact assessment tool, proved to be useful for discovering less tangible and even hidden effects at the individual and group levels of stakeholders both within and outside the PAR setting. We believe that focusing on substantive results only undervalues the richness of the process of participation and its related effects.