Cloud Computing is revolutionizing the Information Technology (IT) industry by enabling organizations to have flexible costs by buying a service instead of owning their assets. Nevertheless, decision makers (DM) in IT organizations have difficulties in evaluating Cloud services because there are no guidelines or any structured form to decide what Cloud services they should use. In this paper, we propose a model based on a set of criteria to evaluate Cloud services that consists in six groups of thirty measurement criteria. In order to evaluate our proposal we have made demonstrations with Google Apps and Microsoft Office 365. We have also made twelve interviews with clients, suppliers, and experts of Cloud services. The results showed that the model was globally accepted and can help the decision-making about Cloud services.