The addition of gas phase H and D atoms to unsaturated hydrocarbons physisorbed on metal surfaces is a viable synthetic route to partially-deuterated alkyl groups on the surface (Jenks, C. J.; Xi, M.; Yang, M. X.; Bent, B. E. J. Phys. Chem. 1994, 98, 2152-2157). Because these processes are exothermic by similar to 60 kcal/ mel, the possibility of alkyl decomposition and/or rearrangement prior to thermal accommodation with the surface (a possibility not explicitly addressed in prior studies) should be considered. In the studies here, these decomposition and rearrangement possibilities have been investigated by studying H and D atom addition to variously-deuterated ethylenes physisorbed on a Cu(100) surface. Ethyl decomposition by C-H, C-D, or C-C bond scission has been addressed and shown not to occur by comparison with results from previous studies of the surface species that would be formed by these bond scission processes. H/D shift between the two carbons of the ethyl groups has been addressed by heating the surface to induce beta-hydrogen or beta-deuterium elimination. The resulting alkene product ratios are compared with those for beta-elimination from selectively-deuterated ethyl groups formed by an independent route, i.e., the dissociative adsorption of a labeled bromoethane. The results show that the extent of H/D shift, if it occurs at all, is <5%. On the basis of this finding and the product ratios, it is determined that the kinetic isotope effect (k(H)/k(D)) for beta-hydrogen/beta-deuterium elimination is 9.5 +/- 0.4 at similar to 260 K on Cu(100). No secondary isotope effect of D for H substitution at the alpha-carbon is detected to within the experimental uncertainty. These results demonstrate, at least for a Cu(100) surface, the feasibility of synthesizing selectively-labeled surface alkyl groups from H and D atom addition to alkenes.