[en] 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 deg. C. 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 deg. C 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