In situ DRIFTS was used to probe the reaction mechanism of steam reforming of methanol over 1% Pt/ceria. Links were drawn between those results and the results of catalytic testing in a fixed bed reactor, including switching studies between deuterated and non-deuterated feeds. The steam reforming mechanism is suggested to proceed via adsorbed methoxy, formate, and carbonate species, and is selective at low conversion. At higher conversions, CO2 reacts via reverse water-gas shift via surface formate intermediates to produce significant levels of undesired CO product. © 2005 Elsevier B.V. All rights reserved.