In this paper we contribute to the literature on government transparency by attempting to provide an answer to the question of what drives in local government transparency levels. Our main objective is to discern whether transparency is mainly driven by the leadership, capacity and other political traits of the local governments themselves ("supply-side determinants") or, on the other hand, is contingent upon social, cultural and/or other exogenous factors ("demand-side" determinants). We test the hypotheses derived from this theoretical framework using the results of the newly developed Municipal Transparency Index (MTI) which was applied to all Portuguese municipalities for the first time in 2013. Preliminary findings indicate that municipalities run by male mayors, with a larger number of consecutive terms in office, and larger margins of victory in local elections display lower levels of transparency. On the demandside, the unemployment rate and the average age of the municipal population emerge as the best predictors and are both negatively associated with transparency.