The mechanism for the reaction of CO coupling to diethyl oxalate (DEO) over Pd-Fe/alpha-Al2O3 in gaseous phase at normal pressure was studied by X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared spectroscopy (DRIRS) techniques. The adsorption of two reactants, carbon monoxide and ethyl nitrite, on the catalyst was observed, respectively with in situ DRIRS as well as the catalysts after ethyl nitrite adsorption and after in situ reaction were also characterized by XPS measurement. These observational results show that (1) The ethyl nitrite on the catalyst surface has a dissociative chemisorption which leads to the oxidation of Pd-0 to Pd2+ (active component on the catalyst). So the reaction is a redox process; (2) Two intermediates, palladium complexes, in the reaction process are involved. From this, the mechanism steps of the reaction were proposed and the two intermediates were inferred. (c) 2005 Elsevier B.V. All rights reserved.