A phenomenological theory applied to a range of solid electrolytes yields both ionic conductivities and excess heat capacities in reasonably good agreement with the experimental. The change in the magnitude of the ionic conductivity appears to be related to the number of available sites per mobile ion, the number of stationary cations (e.g., Hg and Rb in silver electrolytes), and a quadratic interaction energy parameter. The model evidences the possibility of hysteresis, which has been experimentally observed in several of the substances studied.