Resistance measurements and direct spectroscopic investigations were used to monitor the surface reaction path between oxygen and water at the surface of SnO2. The experiments were carried out at high sensor operation temperature (330 and 400 degrees C) and at a constant background of water vapour. We found that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water-oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors. (c) 2005 Elsevier B.V. All rights reserved.