Based on ethnographic studies of two business-to-business (B2B) electronic commerce (EC) marketplaces and a review of interorganizational relationship literature, this paper proposes a model explaining why organizations form alliances to create and why organizations join B2B EC marketplaces. This paper proposes viewing B2B EC marketplaces as a public good. The paper uses public good theory as an umbrella integrating transaction cost and resource dependency perspectives with Oliver’s [1990] determinants of interorganizational relationships.