Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO2 sensors at different temperatures between room temperature and 300 degreesC. The results show the presence of different surface OH groups as well as coordinated water on the SnO2 sensor surface. Their intensity changes with temperature. During the temperature cycles the bands' peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 degreesC at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO2 surfaces, the appearance of gaseous CO2 and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements. (C) 2003 Elsevier Science B.V. All rights reserved.