A solid-state sensor based upon a Na2SO4-NASICON couple surrounded by yttria-stabilized zirconia (YSZ) is shown to respond to changes in P-SO3 independently of P-O2 and P-SO2. In this mode, both electrodes are exposed to the same test gas eliminating the need for a separate reference compartment. This greatly increases the life of the sensor. The same sensor has been successfully adapted to measure SO2 by utilizing a catalyst to convert SO2 to SO3 by a factor which is constant at a given P-O2 and temperature as governed by thermodynamic equilibrium.