Adsorbed cyclohexene can be hydrogenated at cryogenic temperatures by both incident gas phase atomic hydrogen and desorbing bulk hydrogen in the presence of adsorbed surface hydrogen on a Ni(100) surface. In both cases, no C-C bond activation is observed, and cyclohexane is the only hydrogenated product. Cyclohexene desorbs without significant reaction from the Ni(100) surface in the presence of coadsorbed surface hydrogen. Selective hydrogenation of adsorbed cyclohexene by bulk hydrogen is observed at 178 K in the leading edge of the bulk hydrogen desorption peak. Hydrogenation of adsorbed cyclohexene by gas phase atomic hydrogen is observed below 140 K. Isotope studies of the hydrogenation mechanism suggest that the hydrogenation of adsorbed cyclohexene by gas phase atomic hydrogen is a sequential process with the first hydrogen adding from the gas phase and the second from the surface. The small amount of benzene observed from adsorbed cyclohexene monolayers indicates that gas phase atomic hydrogen also causes some hydrogen abstraction.