The chemistry of CH3 radicals on oxygen-modified Mo(100) surfaces (O/Mo(100)) has been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Gas-phase CH3 radicals were produced by pyrolysis of azomethane and dosed on O/Mo(100) at a surface temperature of 320 K. In TPD, O/Mo(100) with theta(O) = 1.4 monolayer (ML) produces exclusively CH4 and CO, but O/Mo(100) with theta(O) = 0.9 and 0.4 ML produce significant amounts of C2+ alkenes in addition to CH4 and CO. HREELS shows that the CH3 groups are bound to surface Mo atoms, not to surface oxygen. On 1.4 ML-O, the CH3 groups are stable at 320 K and have a symmetry lower than C-3v. On 0.9 ML-O and 0.4 ML-O, some CH3 groups decompose to methylene groups, which react with intact CH3 groups to form surface alkyl groups. The surface species at 320 K appear to be controlled by the preadsorbed oxygen coverage, depending on whether theta(O) < 1 ML or theta(O) > 1 ML. CH4 is formed via hydrogenation of CH3 groups by surface hydrogen that is a product of CH3 decomposition. C2+ alkene products are formed by beta-hydrogen elimination of surface alkyl groups. When atomic iodine is coadsorbed on O/Mo(100), the alkene yield in TPD is significantly reduced.