Abstract The high temperature solid electrolyte cell air, M¦ ZrO 2 (8% Y 2O 3 )¦M , air-SO 2SO 3, where M is Pt, Ag, or Au, was used to monitor the oxygen activity on Pt, Ag, or Au catalyst films exposed to mixtures of O 2, SO 2, and SO 3 at temperatures above 400 °C and 1 atm total pressure. One electrode functioned simultaneously as both electrode and catalyst. The opencircuit EMF of the cell reflected the oxygen activity on the working catalyst and simultaneously the steady-state chemical kinetics were observed. For Pt it was found that the surface oxygen activity generally does not equal the gas phase P O 2 , but is determined by P SO 2 or the P SO 3 P SO 2 ratio, depending on the temperature and gas phase composition. These observations suggest that the rate-limiting step in the SO 2 oxidation on Pt is not generally the reaction between chemisorbed oxygen and gaseous SO 2, except at high temperatures and very low P so 2 , but rather the desorption of a chemisorbed phase of SO 3 or the adsorption of oxygen, depending upon the temperature, P SO 2 , P SO 3 , and P O 2 . The results with Au and Ag films show that chemisorbed SO 3 is also formed on these metals exposed to SO 2, O 2 mixtures. The values of surface oxygen activity compared to those obtained for Pt, provide an explanation for the weakly catalytic and noncatalytic properties of Au and Ag, respectively, for the SO 2 oxidation.