Colloidal nickel was easily prepared by reaction of nickel bromide with zinc powder. It was an active catalyst for hydrogenation of carbon-carbon double bonds. Styrene homologs were hydrogenated to afford the corresponding alkanes quantitatively in the presence of the colloidal nickel under an atmospheric pressure of hydrogen. A plot of the volume of hydrogen uptake vs. time gave a straight line. The hydrogenation rates of styrene homologs were compared with that of styrene. The steric and electronic factors of substituent on the double bond affected the rate of hydrogenation significantly.