This work describes the construction of an experimental setup — a mass spectrometer coupled microreactor — which makes possible the simultaneous monitoring of the electric resistance of a solid sensor sample and the composition of the near surface gas atmosphere. The information obtained from the constant voltage resistance measurement is complemented by the information about of the near-sensor gas composition. The evaluation of both sets of results offers insight into the mechanism of the sensing process. As an example, the results obtained on an undoped, oven-heatedCeO2sensor sample is presented, in a CO and oxygen containing atmosphere between 25 and 900 °C. It was found that between 200 and 300 °C the decrease in the sensor resistance is mainly due to the CO chemisorption. In this temperature range the oxidation of CO is negligible. The CO sensitivity decreases with the temperature in the whole interval examined. However, its value is the highest in this temperature range due to the relatively high surface coverage. In the 300–400 °C range the most important process is the gas phase oxidation of the CO; about 85&percent; of it is transformed. The transformation of CO leads to a smaller surface coverage, and so to a decreasing — but still high enough — sensitivity. A further temperature rise hinders the CO chemisorption and the lack of chemisorption results in no CO sensitivity. It is noted that even the catalytic effect of theCeO2sample is important in the studied system; its effect on the near-sensor gas composition is easily detectable. The maximum catalytic activity ofCeO2in the CO oxidation was found to be around 400 °C. ©1998 American Institute of Physics.