The addition reaction of gas-phase D atoms to olefin monolayers adsorbed on a Cu(100) surface is studied, with a focus on the regioselectivity of deuterium addition onto monolayer 1-butene and 1-pentene molecules. Both 1- and 2-alkyl groups are generated from D atom additions to 2 degrees and 1 degrees carbons, respectively. The alkyl groups are separated based on a difference in their beta-hydride elimination kinetics, with the rate of 2-alkyl groups losing beta-hydrogens about 2 orders of magnitude faster than 1-alkyl groups on the copper surface. The results suggest that D addition to terminal (1 degrees) carbon is favored by a factor of similar to 3 for 1-butene and similar to 4 for 1-pentene molecules adsorbed on the surface.