Aim of this paper is to analyse electronic marketplaces from an organisational point of view. The marketplaces are herein considered as a particular form of electronic network and are analysed in the light of transaction cost theory. Besides considering the three classical effects identified by Malone et al. (the communication effect, the electronic integration effect, the electronic mediation effect), the paper, on the ground of empirical evidences, also evaluated a fourth effect defined by Wigand as “the strategic electronic network effect”. Analysis two cases the paper describes how ICT affects the market organisation of marketplaces and the relationships among the actors that are involved in this particular electronic networks.