The interaction of gaseous D atoms with methyl iodide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied. Direct product rate measurements were employed to determine the reaction kinetics. On all substrates, incoming D atoms abstract the methyl group from adsorbed CH3I via gaseous CH3D formation. in the monolayer regime of CH3I/Pt(111) pure hot-atom phenomenology was observed in the rates. With multilayers as targets, the fluence dependence of the rates get Eley-Rideal-type. With a coadsorbed H monolayer present, the CH3D rates at a CH3I monolayer on Pt(111) are affected by the suppression of hot-atom sticking. Accordingly, the rate curves exhibit similar features as expected for Eley-Rideal phenomenology. However, CH4 as a product and simultaneous abstraction of adsorbed H via gaseous HD and H-2 formation clearly demonstrate that hot-atom reactions occur. With CH3I adsorbed on a graphite monolayer on Pt(111), the abstraction kinetics of methyl was found to agree with the operation of an Eley-Rideal mechanism. This observation is in line with the expectation that hot atoms do not exist on a C/Pt(111) surface.