Environmental purchasing has been one of its most significant elements in Green Supply Chain Management (GSCM). By implementing environmental purchasing, companies adopt additional criteria for evaluating suppliers, which, as argued, gen­erates additional transaction costs. From the GSCM theoretical basis, and looking through the analytical lenses of Transaction Cost Economics (TCE), this article aims to discuss the transaction costs involved in the supplier selection process with the environmental purchasing approach. As result, this article presents five propositions of TCE within the GSCM in the light of two cases, which relate the possible transac­tion costs involved in each stage of the environmental purchasing process and also according to the main transaction elements.