Apart from the difficulty to attract new members, leakage of sales outside the cooperative is a major challenge for the coffee cooperatives in Rwanda. Local (independent) traders still constitute a major market for coffee producers. Yet, cooperatives also accept the produce from non-members and pay them the same price. Our objective in this paper is to analyse the importance of this phenomenon of double side-selling. We collected data from a sample of 170 coffee farmers. We use a probit model to analyse characteristics linked to cooperative membership and to study double side-selling.