A plasticity model involving dilatancy and stress-induced dissolution of crystalline material has been formulated to qualitatively account for the high temperature constant strain-rate and steady state creep data obtained for a glass-ceramic derived from the SiO=Li20-ZnO system and reported previously. To test the model further, high temperature stress relaxation experiments have been carried out on the same material. Also, by altering the concentrations of minor constituents, including the nucleating agent, it has been possible to examine the effects on mechanical properties of both the grain size and the viscosity of the non-crystalline material.