In this study, we developed a model to explain electronic-procurement systems (EPS) adoption, considering the technology-organization-environment framework as well as the institutional theory. This model was tested with data collected from the 2500 biggest companies operating in Portugal. Based on the t-test for equality of means we found evidence that EPS adoption is positively and significantly related to (1) firm size, (2) technological capabilities,(3) the perception companies have about the EPS success of their competitors, (4) the extent of adoption among competitors and (5) the trading partner readiness to perform electronic transactions. The logistic regression supplied further evidence that technological capabilities, firm size, extent of adoption among competitors and trading partner readiness provide a reasonable estimate for each firm’s likelihood to adopt EPS. We also found evidence that firms which main activity is commerce are more likely to adopt EPSs than companies from manufacturing or services industries.