Recent studies have shown that adsorbed alkyl groups can be generated and isolated on metal surfaces by a variety of methods, including the thermal dissociation of alkyl iodides. For copper surfaces, the primary reaction pathway is beta-hydride elimination at similar to 250 K to produce the corresponding olefin and adsorbed hydrogen atoms. We show here that, on a Cu(100) surface, 1-10% of the product olefin is rehydrogenated to the alkyl group. This reversibility has been detected and quantified by deuterium isotope labeling in temperature-programmed reaction experiments. In addition, partially deuterated ethyl groups have been synthesized on the surface by a novel hydrogenation reaction, and these selectively deuterated alkyls have been used to measure the deuterium isotope effect for beta-hydride elimination which is 10.3 +/- 0.7 at 255 K. From these results, the enthalpy change for beta-hydride elimination and the metal-ethyl bond energy on Cu(100) have been determined to be 6.5 +/- 4 kcal/mol and 33 +/- 6 kcal/mol, respectively. These values are compared with those for alkyl groups on other metal surfaces, for alkyl groups in metal compounds, and for alkyl groups bonded to metal atoms in the gas phase.