The study presented in this paper seeks to understand if the payment systems can be considered as a barrier for the B2c E-Commerce diffusion. More in detail the objectives of the paper are two-fold: to give a detailed picture of the payment systems used to shop online and of the main related issues both from the merchant and the consumer side; to build an explanatory theory to justify the current and future diffusion of the various payment systems that takes into account the factors influencing their suitability for the B2c E-commerce. The analysis is mainly based on case studies among the most important (top100) Italian E-commerce merchants which have been carried out as part of the B2c Observatory of the School of Management of Politecnico di Milano. The latter has studied the online sales in Italy since 2000 and has activated a research stream focused on the payment systems supporting the B2c E-commerce transactions.