The influence of SO2 on the output of resistive oxygen sensors using CeO2 and Ce0.8Zr0.2O2 thick films was investigated. At 873 and 1073 K, the resistance of the sensors using CeO2 film was strongly influenced by 1, 500 ppm SO2, and SO2 caused a decrease of resistance. On the contrary, the resistance of the sensors using Ce0.8Zr0.2O2 was hardly influenced. The resistivity of the reaction product, which was Ce-2(SO4)(3), was 100 times greater than that of CeO2 at 873 K. Thermodynamic calculation revealed that CeO2 was more stable than Ce-2(SO4)(3) at 1073 K in 500 ppm SO2. These results showed that the resistance decrease of the sensor using CeO2 film, caused by the introduction of SO2, was unrelated to the formation of the reaction product. It was believed that the resistance decrease of the sensor using CeO2 film was due to the adsorption of electrically conductive substances on the CeO2 surface. It was considered that the reason the influence of SO2 on Ce0.8Zr0.2O2 thick film was less than that on CeO2 thick film was that the resistivity of Ce0.8Zr0.2O2 was 1/10 times that of CeO2. (c) 2005 The Electrochemical Society. All rights reserved.