The effect on thermal properties and non-isothermal devitrification of substituting CaO by Y2O3, in a glass of composition 2.5 CaO.2SiO(2) was studied. The glass samples were submitted to thermal analysis (DTA) and X-ray diffraction analysis. The trends with the composition of the glass transformation, T-g, softening, T-s temperature and the activation energy of crystal growth, E-c, can be explained on the basis of the increase of the structural rigidity when Ca2+ ions are substituted by Y3+ ions; stronger bonds to the oxygen are, indeed, expected. In all the devitrified samples CaO.SiO2 crystals form together with a secondary calcium silicate phase. The pattern of the Y2O3 free glass shows that the alpha CaO.SiO2 is also formed in the temperature range 900-1000 degrees C. When x = 1.0 a new, crystalline phase appears, which could not be identified by means of JCPDS cards. The activation energy for crystal growth was E-c = 640 +/- 25.1 kJ/mole, regardless of composition. Devitrification involved a mechanism of surface nucleation : however, as the samples softened and sintered before devitrifying, surface nuclei behaved as bulk nuclei.