Combining the findings obtained by different research methods in mixed-research synthesis could potentially contribute to a broader, more diverse evidence base for interventions. In this article we focus on the methodological challenges involved in synthesizing various types of research findings. We propose a method that uses hypotheses to facilitate the comparison and integration of such different findings. The method consists of four steps: (1) synthesizing findings per source of evidence, (2) formulating a mono-method hypothesis for each source, (3) integrating the monomethod hypotheses into one overall hypothesis, and (4) evaluating, using empirical data, whether the overall hypothesis better fits the data than each of the mono-method hypotheses. Using quantitative studies, qualitative studies and experts’ views in the substantive case of children and trauma, we will illustrate the proposed method. We conclude that the method provides a viable perspective for constructing an elaborate model that captures the knowledge from complementary sources.