In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFT'S) was used to clarify the reaction mechanism of a new methanol synthesis method on a Cu/MgO catalyst at 443 K from syngas containing CO2 using an ethanol promoter. The adsorbed formate species were formed by exposing Cu/MgO catalyst to syngas (CO/CO2/H-2), and it reacted easily with ethanol to form ethyl formate in two states: gas-phase species and physisorbed species, at low temperature. The ethyl formate was the reactive intermediate, and it was reduced easily by hydrogen atoms on Cu to form methanol at low temperature. The reaction temperature was significantly decreased due to the promotional catalytically active action of ethanol. This is a new reaction route, as ethanol used in this reaction was not consumed due to its self-regeneration by hydrogenation of ethyl formate. In order to accelerate this reaction, one must introduce a large amount of ethanol into the reaction system. (c) 2005 Elsevier B.V. All rights reserved.