The coadsorption of methanol and oxygen on Cu(100) has been examined using secondary ion mass spectrometry (SIMS). The study provides a mass-spectrometric observation of the surface intermediates present during adsorption at 130 K, and after heating to 320 K. Isotopically-labelled reagents (O-18(2), CD3OD. CH3OD) have been used to facilitate the interpretation of spectra, The SIMS data indicate that MeOD dissociates at 130 K in the presence of pre-adsorbed O-18(2) to yield a physisorbed D2O layer, in addition to a methoxy (MeO) intermediate and other products. Spectra obtained at 320 K reveal an enhanced concentration of formats species (predominantly as (HCOO)-O-18-O-16). The principal positive secondary ion associated with the methoxy intermediate was found to be CuCH2O+, rather than the expected CuCH3O+;, which was not detected. The observed behaviour is consistent with reaction mechanisms proposed by Wachs and Madix to account for methanol oxidation on O/Cu surfaces [J. Catal, 31 (1982) 243].