We have compared estimates of the activation volume V* based on several theoretical models with measured values in metals, alkali halides, and olivine. The theoretical methods tested include one based upon the empirical correlation between activation energy and melting temperature and several which are based upon simple elastic models for the defect structure. For metals and olivine, the melting relation works well, but for alkali halides, the melting model predicts too large a V* by an approximate factor of 1.5. Of the elastic models, the dilatational strain energy model introduced by Zener (1942) provides reasonable estimates of V* for metals and olivine, but it also overestimates V* for alkali halides. Zener's assertion that the experimental value of V* should be bounded by theoretical values calculated from strain energy models which assume pure shear (Keyes, 1963) and pure dilatation is supported by the available data for metals, oxides, and alkali halides. These provide upper and lower estimates for the variation of viscosity with depth in the mantle.