Gas phase hydrogen atoms add to adsorbed cyclohexene at 100 K on the Ni(100) surface, resulting in cyclohexane formation during subsequent TPR (temperature-programmed reaction) experiments. No C-C bond activation is observed after exposure to gas phase atomic hydrogen. Vibrational and isotope studies indicate that a cyclohexyl intermediate is formed by the addition of gas phase atomic hydrogen to adsorbed cyclohexene. This adsorbed cyclohexyl is hydrogenated primarily by surface hydrogen to form cyclohexane during subsequent heating. Cyclohexane yields increase with increasing atomic hydrogen exposure, and yields in the 90% range have been observed with large atomic hydrogen exposures. In the presence of only coadsorbed hydrogen, no significant hydrogen addition to adsorbed cyclohexene is observed, and cyclohexene desorption dominates during subsequent TPR experiments. Vibrational spectroscopy indicates that pi-bonded cyclohexene with the C=C bond parallel to the surface is the dominant surface species in the presence of coadsorbed surface hydrogen. In contrast, di-sigma-bonded cyclohexene is the dominant species in the absence of coadsorbed hydrogen. In the absence of coadsorbed hydrogen, dehydrogenation of the adsorbed cyclohexene results in benzene formation with increasing temperature. Adsorbed benzene formed by dehydrogenation has been identified after heating to 390 K in the absence of hydrogen using vibrational spectroscopy.