The purpose of this research is to investigate economic factors that determine characteristics of B2B e-Marketplaces. From a previous research work, we classify B2B e-Marketplaces into four forms: MRO Hubs, Catalogue Hubs, Yield Managers, and Exchanges. This classification is based upon "What businesses buy" and "How businesses buy." In an attempt to identify economic factors that determine e-Marketplace forms, we employ three economic factors: buyer/seller relationships, features of products traded, and characteristics of industries or markets. Through extensive literature reviews, we have selected six variables which can be used to explain reasons for different e-Marketplace forms: asset specificity and information asymmetry for the buyer/seller relationship, standardization and price variability for the product features, entry barrier and market volatility for the market characteristics. This research conducts six in-depth case studies to explain that "Why different forms of e-Marketplace emerge?" The research framework intends to provide link between e-Marketplace forms and six economic variables. Six case studies are based on interviews with CEO or team managers of e-Marketplace firms. This study has found six economic variables, explaining well reasons for different e-Marketplace forms. Research findings are summarized as propositions so that survey-type research can be conducted with a large number of samples in the future.