E-procurement systems make purchasing activities more effective in terms of both time and cost. However, over the past years there is evidence that some of the expected benefits have not been achieved. Among several causes, supplier's low adherence to such platforms has been regarded as one. The focus of this research is in supplier adoption of e-Procurement. It will help to better address the issues actually faced by suppliers within e-Procurement. We have conducted a questionnaire-based survey to 721 Portuguese companies and performed an empirical analysis of the data. The findings from this work provide evidence that the supplier perceived indirect benefits and business partner pressures are most important to e-Procurement adoption while barriers have a negative impact on their adoption. The main critical success factors on e-Procurement adoption are also presented.