Despite the high interest from many researchers on B2B e-marketplaces, there has been a limited empirical investigation on their post-adoption stage. Based on literature dealing with their e-business model and building on emerging concepts in B2B e-commerce, this paper aims at assessing the influence of various factors on firms’ level of use of B2B e-marketplaces. Specifically, three basic domains are investigated: a) firms’ internal environment, b) their external environment and c) the characteristics of the B2B e-marketplace. A conceptual framework is developed and examined on data collected from 112 firms that currently use Greek B2B e-marketplaces. These are analyzed through factor analysis and multiple discriminant analysis. Findings show that factors from all the categories examined impact on firms’ level of use with the domain concerning the characteristics of the B2B e-marketplace having the highest influence. The results also provide interesting insights and useful hints to both researchers and practitioners.