In this paper Tg values of calcium (sodium) silicate glasses containing added with oxides of trivalent elements are reported. The plots of Tg as a function of composition or vs. the ionic field strength prove to be useful in discussing the role of the oxides in the glass structure. It is found that, at least in the studied composition range, Sc2O3, Y2O3, La2O3, and In2O3, behave as network modifier oxides. In the compositional ranges studied, the hypothesis based on them well agree with the expectations based on the known criteria reported in literature and on FTIR spectra.