The paper investigates the gas response of nanocrystalline SnO2 based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H2O) and oxygen (O-2) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO2. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R-0 and the resistance in the presence of the target gases, R, namely R-0/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O-2 and H2O related surface species and, as a result, different sensing mechanisms can be observed for CO. (C) 2003 Elsevier Science B.V. All rights reserved.