The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO2 reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H-2 enhances the CO oxidation rate on Au/TiO2 without leading to any major loss of selectivity. Increasing the H-2 pressure induces higher CO and H-2 oxidation rates. Under H-2-free conditions, the surface species detected are Au delta+-CO, Ti4+-CO, carbon dioxide and carbonates. Upon the addition of H-2, Au-0-CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving HxOy species under the present experimental conditions is proposed. (C) 2008 Elsevier B.V. All rights reserved.