Sterically hindered olefins can be hydrogenated at -50 degrees C in dichloromethane using triflic acid (CF3SO3H) and a hydride donor. Mechanistic studies indicate that these reactions proceed by hydride transfer to the carbenium ion that is formed by protonation of the olefin. Olefins that form tertiary carbenium ions upon protonation are hydrogenated in high yields (90-100%). Styrenes generally produce lower yields of hydrogenated products (50-60%). Suitable hydride donors include HSiEt(3) and several transition metal carbonyl hydrides (HW(CO)(3)Cp, HW(CO)(3)Cp*, HMo(CO)(3)Cp, HMn(CO)(5), HRe(CO)(5), and HOs(CO)(2)Cp*; Cp = eta(5)-C5H5, Cp* = eta(5)-C(5)Me(5)). A characteristic that is required for transition metal hydrides to be effective is that the cationic dihydrides (or dihydrogen complexes) that result from their protonation must have sufficient acidity to transfer a proton to the olefin, as well as sufficient thermal stability to avoid significant decomposition on the time scale of the hydrogenation reaction. Metal hydrides that fail due to insufficient stability of their protonated forms include HMo(CO)(2)(PPh(3))Cp, HMo(CO)(3)Cp*, and HFe(CO)(2)Cp*. Other hydrides that fail are those that are protonated to give dihydrides or dihydrogen complexes that are not sufficiently acidic to protonate olefins, as found for HW(CO)(2)(PMe(3))Cp and HRu(CO)(PMe(3))Cp.