The effect of coadsorbed oxygen on the thermal chemistry of iodomethane on Ni(110) single-crystal surfaces was studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. It was found that extensive molecular desorption occurs at temperatures approximately 60 K higher than those seen for the desorption of multilayer CH3I on the clean surface, and that the activation of the C-I bond in the adsorbed iodomethane is deferred but not fully eliminated by the coadsorbed oxygen. The selectivity for the conversion of the surface methyl intermediates produced by the C-I bond scission is also affected by the oxygen precoverage. In broad terms, a decrease in methane production is accompanied by the formation of heavier hydrocarbons, mainly ethane and ethylene but also some propene and butene. Most of these hydrocarbons are presumably produced via a mechanism centered on a methylene insertion step, but additional low-temperature (similar to190 K) ethylene and ethane are also made by direct coupling of methylene and methyl groups, respectively. Small amounts of formaldehyde, the only oxygenated hydrocarbon observed, and propenyl and butenyl radicals are seen as well.