This study presents a quasi-experimental analysis of the impact of FairTrade certification on the commercial performance of coffee farmers in Tanzania. In doing so the study emphasises the importance of a well-contextualised theory of change as a basis for evaluation design. It also stresses the value of qualitative methods to control for selection bias. Based on a longitudinal (pseudo-panel) dataset comprising both certified and conventional farmers, it shows that FairTrade certification introduced a disincentive to farmers’ commercialisation.