A new SO2 gas sensor based upon a magnesium ion conducting solid electrolyte MgZr4(PO4)(6) (MZP) and a Na2SO4 auxiliary electrode has been fabricated and tested. Electromotive force (emf) results with variation in the SO2 and the O-2 partial pressures at various temperatures from 873 to 1023 K are presented. The results have shown that the response to SO2 is rapid and the measured emf of the sensor is found to vary linearly as a function of the logarithm of the partial pressure Of SO2. As MZP does not react with SO2, both electrodes of the sensor can be exposed to the same atmosphere containing SO2 and thus there is neither any need for sealing the electrode compartment nor using any semi-permeable membranes.