Hydrogen spillover phenomena on various supported metal catalysts were studied by means of hydrogen chemisorption and vapor phase hydrogenation of benzene. Enhancement of the adsorption of hydrogen by increasing the amount of diluents in contact with the catalysts was the result of migration of the spillover hydrogen from the metal to the support and diluent. However, the reaction activity increased with the degree of dilution of the catalysts with the diluents and then decreased in more diluted systems. The results obtained could be interpreted by the double spillover effect, the migrating distance and the recombination of hydrogen on the supports and diluents by collision. The reaction activity was influenced by the dilution ratio and the reaction temperatures, but the activation energy and the reaction mechanism did not change even if hydrogen spillover occurred.