In this paper the non-isothermal devitrification of a glass of composition CaO . 0.1P(2)O(5) . 0.9SiO(2) has been studied. Comparison with literature data about the glass CaO . SiO2 enables discussion of the influence of the equimolar substitution of P2O5 for SiO2. The crystal growth activation energy is E(c) = 128 +/- 14 Kcalmol(-1) and is lower than that for CaO . SiO2. The experimental results suggest that this effect can be linked to an analogous compositional effect on the viscous how activation energy. A surface nucleation mechanism is operative as in the case of the parent glass CaO . SiO2. However the nucleation rates would be reduced as a result of the need for diffusion in the non-stoichiometric glass.