Mechanistic aspects of steam reforming of methanol were studied via steady-state isotopic transient kinetic analysis over three copper-based catalysts, namely combustion-synthesized Cu-Ce-O and Cu-Mn-O, and commercial Cu-ZnO-Al2O3. The "C-path" and "O-path" for the production of CO2 via steam reforming of methanol was analysed with the following step changes in the feed: (CH3OH)-C-12/H2O/Ar/He -> (CH3OH)-C-13/H2O/He and CH3OH/(H2O)-O-16/Ar/He -> CH3OH/(H2O)-O-16/(H2O)-O-18/He. The presence of (CH3OH)-O-18 in the products after the switch to O-18-labeled water indicates that a major path of the reaction is the one involving a methyl formate intermediate. This appears to be the main path over the Cu-Mn-O catalyst, while parallel paths via dioxomethylene and methyl formate intermediates appear to be operative over Cu-Ce-O and Cu-ZnO-Al2O3 catalysts. (C) 2008 Elsevier B.V. All rights reserved.