Hydrogenation of 1-hexene and cyclohexene was studied over silica-supported Pt, Pd and Ph catalysts in the liquid phase, in the presence of quinoline or carbon tetrachloride at room temperature. As the amount of surface modifiers increased in the reactor, the rate of hydrogenation decreased over Pd/SiO2 and Rh/SiO2. The activity of Pt/SiO2 could also be lowered with quinoline; however, it remained constant for cyclohexene hydrogenation or decreased just slightly for l-hexene even when excess CCl4 was supplied. The inhibition by CCl4 was substantial for palladium and rhodium; nevertheless, complete deactivation did not occur. Quinoline effectively deactivated the palladium and rhodium catalysts in cyclohexene hydrogenation, but residual activity for l-hexene hydrogenation was above 50% for each of the catalysts