The kinetics and mechanism of the preferential oxidation of carbon monoxide in the presence of hydrogen (PrOx) over an unsupported gold powder (mean particle size similar to 20 nm and free of silver) have been investigated using How fixed bed catalytic testing and diffuse reflectance infrared Fourier transform spectroscopy coupled to mass spectrometry (operando DRIFTS or DRIFTS-MS). It is shown that the presence of H-2 has a favourable effect on the oxidation of CO, either by strongly accelerating the reaction or by preventing the catalyst deactivation, depending on the conditions used. Variation of the hydrogen partial pressure has allowed us to determine partial reaction orders for both CO oxidation and H-2 oxidation under PrOx conditions. An infrared band at similar to 2113 cm(-1), corresponding to on-top CO adsorption on metallic gold, has been observed below 150 degrees C. In addition, adsorbed hydroxyl groups gradually develop simultaneously to gas-phase water in the course of the reaction at increasing temperatures. The promotional effect of hydrogen is ascribed to highly oxidative HxOy intermediates formed from the interaction between H-2 and O-2 on the gold surface. (C) 2008 Elsevier B.V. All rights reserved.