The effect of water on the formation and stability of surface species in ethanol adsorption on TiO2 and Rh/TiO2 was studied by FTIR. The changes of the gas phase during the adsorption processes and different treatments were monitored and TPD spectra were registered by mass spectrometer. It was found that water enhanced the stability of strongly held C2H5OH, while the stability of monodentate ethoxide was less due to water. Dehydrogenation of molecularly adsorbed ethanol resulting in the formation of adsorbed acetaldehyde and gas phase H-2 have been proposed as key reaction step. Surface acetaldehyde and gas phase H-2 appeared at lower temperature and in a greater amount in the presence of Rh and water. A possible mechanism of the reaction was discussed. (C) 2004 Elsevier B.V. All rights reserved.