This paper proposes, in the context of Open Government, a model to assess how public sector entities are taking advantage of the Internet as an information disclosure tool and a means to promote transparency, specifically in what concerns the use of public resources (input transparency for accountability). The assessment model and resulting Transparency Index gives particular attention to the disclosure of detailed (disaggregated) data according to the principles of Open Government Data, namely by valuing data visibility, adequate format for further processing, and the autonomous presentation of individual information items. Subsequently, the paper demonstrates the applicability of the proposed model by carrying out two assessment exercises on two subsets of Portuguese and Italian municipalities. Results show that, all in all, the municipalities analysed do not yet disclose enough information useful for accountability processes and they do not take advantage of the Internet potential to make the data provided more visible and re-usable by citizens and local stakeholders. Alone, high-level policy directives, governmental requirements and national legislation guaranteeing access to information are not enough to ensure public entities (municipalities in particular) disclose all the relevant data, and therefore specific guidelines are needed.