The fate of hydroxyls at the surface of gamma-Al2O3 supported palladium oxide catalysts under lean methane combustion reaction conditions was investigated by in situ diffuse reflectance-fourier transform infrared (DR-FTIR) spectroscopy. Direct evidence was obtained for the formation and accumulation of hydroxyls at the catalyst surface during methane oxidation to CO2 and water. Several well-resolved absorption bands were observed in the wave number region characteristic for isolated OH groups along with a wide absorption at a lower frequency assigned to hydrogen bond associated hydroxyls at the surface. Experiments conducted with wet air pulses over the catalyst and the alumina support suggested the observed hydroxyls are related to the presence of the PdO phase at the catalyst surface. Analysis of DR-FTIR spectra recorded at different temperatures suggested the dehydroxylation mechanism proceeds through conversion of multibound to bridged and terminal species before recombination and desorption as gas phase water molecules. The proposed dehydroxylation mechanism has been confirmed by transient temperature experiments. In situ investigations of zirconia and ceria-zirconia supported PdO catalysts suggested that the oxygen mobility of the support influences the dynamic behavior of hydroxyls at the surface. (C) 2003 Elsevier B.V. All rights reserved.