Abstract The compositions (1 - x )Ag 2SO 4-( x)CaSO 4 ( x = 0.01 to 0.20) are prepared by slow cooling of a melt and also by quenching using a twin roller technique. The solid solubility limit of Ca 2+ is determined using X-ray powder diffraction and scanning electron microscopy. Conductivity studies, using a detailed impedance analysis, are carried out in low as well as high temperature regions encompassing both the structural modifications of Ag 2SO 4. In the high temperature region ( T > 417°C), the conductivity is found to be maximal at a 7% vacancy concentration of CaSO 4 added to Ag 2SO 4. However, in the low temperature orthogonal phase, a decrease in conductivity within and above the solid solubility limit is observed. The concept of lattice distortion (contraction) due to partial substitution of Ag + by Ca 2+ is introduced to explain a decrease in conductivity in the orthogonal phase. Interfacial reactions and surface defect chemistry are applied to understand the fall in conductivity in CaSO 4 dispersed in Ag 2SO 4.