Government agencies all over the world are making big investments for developing information systems that open important data they possess to the society, in order to be used for scientific, commercial and political purposes. It is important to understand what value they create and how, and at the same time - since this is a relatively new type of information systems (IS) - to identify the main improvements they require. This paper contributes in this direction by presenting a methodology for determining the value generation mechanism of open government data (OGD) systems and also priorities for their improvement. It is based on the estimation of a ‘value model’ of the OGD system under evaluation from users’ ratings, which consists of several value dimensions and their corresponding value measures, organized in three ‘value layers’, and also the relations among them. The proposed methodology has been successfully applied to an OGD system developed as part of the European project ENGAGE (‘An Infrastructure for Open, Linked Governmental Data Provision towards Research Communities and Citizens’), and provided interesting insights and improvement priorities. This first application provides evidence that our methodology can be a useful decision support tool for important ODG systems investment, management and improvement decisions.