The a.c. electrical conductivity of Ag2SO4 has been measured as a function of pressure up to 1.8 GPa and in the temperature range between 300 K and 1100 K. The data show a decrease in sigma with P and a sharp jump in sigma, becoming more gradual with P, at the orthorhombic --> hexagonal phase transition. The data also suggest an unreported possible phase between the established orthorhombic and hexagonal phases. The activation energy for ionic conductivity for the hexagonal phase is effectively independent of P up to approximately 1.0 GPa where there is a precipitous drop. The activation volume is strongly T-dependent for both the orthorhombic and hexagonal phases. These results are compared to results reported in parallel studies on Na2SO4, AgTlSO4 and other non-sulfate systems. The contrasting conductivity behavior of isomorphic compounds in these systems is discussed in terms of the various factors, including non-structural factors, contributing to fast ion conductivity.