There is a call for more transdisciplinary (TD) research, from academia, society, and funding agencies. Consequently, the field of TD research is searching for ways of proving the value and providing evidence to support the effectiveness of such research. The main challenge for evaluating TD research is attribution, that is how to link societal change to the TD research process. However, little attention has been paid to the relationship between the quality of the research process and the effects and impacts that are being evaluated. Building upon earlier attempts at evaluating TD research, this article tests three key aspects of effective sustainability research: its relevance, credibility, and legitimacy. To explore the link between the quality of process and societal effects, we analyze and compare outputs, outcomes, and impact of five TD projects. Overall, our analysis shows that while relevance, credibility, and legitimacy gave important insights regarding the links between process and impacts, they are not adequate for evaluating TD research impact. Process qualities such as practitioner motivation and perceived importance of the project, together with breadth of perspectives, the openness/flexibility of participants, and in-depth exchanges of expertise and knowledge, contributed to producing internally relevant, credible, and legitimate results. However, we also saw a need to develop the relevance, credibility, and legitimacy framework, in relation to the external dynamics of the project process, heterogeneous stakeholder groups, and the credibility of practice-based knowledge, which together with institutional factors and the political context significantly shape the possibility of impact.