To improve understanding of the deformation properties of subfreezing polycrystalline glacier ice and, in particular the role of crystal size, chemical impurities, and rock particle impurities, we analyze in situ strain rates of the basal layers of Meserve Glacier, Antarctica. Strain rates were monitored on the walls of a subglacial tunnel (where down flow shear stress is relatively uniform) and ice properties were measured (texture, fabric, and impurity content). We propose a simple empirical model describing strain rate variations due to variations in crystal size and impurity content, and we use all relevant Meserve data to constrain model parameters. We conclude that there is a direct dependence of strain rate on crystal size, which reflects an important role for a grain-size-sensitive deformation mechanism such as grain boundary sliding or diffusion. Chemical impurities are found to enhance the grain-size-sensitive deformation and are found to be an important control on strain rate variations in the very impure ices of Meserve Glacier. However, the per molar sensitivity of strain rate to chemical impurity content is shown to be very low, such that in the ice age ices of the Greenland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement, though we cannot exclude chemical enhancements as high as 1.3 there. Our analyses detect no direct rheologic effect of rock particles in the Meserve ices, which suggests that rock content is not directly responsible for the low viscosity of dirty basal layers. ¿ 2000 American Geophysical Union