Given the increasing importance that is attributed to social capital in cooperative-type organizations, we test the hypothesis that members of high-performance cooperatives exhibit higher levels of social capital (as measured by interpersonal and institutional trust) compared to members of low-performance cooperatives. A field survey and three behavioural games were undertaken amongst members of two high- and two low-performance primary marketing coffee cooperatives in the Sidama region in Ethiopia. We find evidence supporting our hypothesis, with members of high-performance cooperatives exhibiting significant higher levels of institutional trust (stated trust and cooperativeness), and interpersonal trust (behavioural trust and trustworthiness) than members in low-performance cooperatives. Furthermore, gender comparisons show that female members have a higher propensity to adopt pro-social behaviour than their male counterparts, in experimental settings. Since the income of members of high-performance cooperatives is considerably higher than the income of their peers in low-performance cooperatives, we conceptualize a relation between social capital, cooperative performance and members’ income as a loop, with feedbacks occurring between these components.