This article investigates commitment in cooperative marketing relationships. Sideselling by members poses a serious threat to the viability of cooperatives, since services provided to members have to be financed through collective sales. We develop a model that demonstrates how the size of the producer determines the degree of individual commitment under the provision of public and private collective goods. Based on survey data from cooperatively organized coffee farmers in Costa Rica, we find that smaller and larger farmers are more committed, while medium-sized farmers side-sell a larger share of their produce to private buyers. Some broader implications for collective marketing relationships are discussed.