Thin films generally consist of compact grains and voids. Their mass density p and refractive index it depend on the related values rho and n, of the grains and the void fraction. A procedure is presented for evaluating the unknown parameters n, and rho, from (n,rho)-pairs measured on films with various densities. The procedure is based on an effective-medium theory and is consistent with the Clausius-Mosotti relation. The underlying microstructural models comprise grains separated by pores. The approach is applied to literature data for TiO2 films prepared by various deposition methods. The grains in sputtered and ion-plated films exhibit anatase properties. A three-phase mixture of rutile, anatase and pores is unlikely. The evaluation for the set of films prepared by filtered arc deposition is consistent with the reported microstructure, i.e. a mixture of statistically oriented rutile grains. The structure of reactively evaporated amorphous films is best described as consisting of grains with a density less than anatase, by 3 and 7.5%, respectively, for two different evaporation processes.