To elucidate the characteristics of rare earth metallic catalysts the hydrogenation of unsaturated hydrocarbons (ethene, propene, 1-butene, 1,3-butadiene, ethyne, propyne, and benzene) were carried out around ambient temperature using samarium and ytterbium particles formed by clustering metal atoms in frozen organic matrices by metal vapor techniques. In the hydrogenation reactions the rare earth metallic catalysts discriminated between the CC double bonds and triple bonds; alkenes, dialkenes, and aromatic compounds were readily hydrogenated, whereas alkynes were not hydrogenated at all. However, enhanced isomerization activity of propyne to propadiene was observed. The addition rates of hydrogen to alkenes were represented on coordinates of a first-order equation: v = kP H 2. Preliminary kinetic studies suggest that the reaction is controlled by the hydrogen adsorption process. This identification is reinforced by the H 2-D 2 isotope scrambling measurements. The hydrogenation of 1,3-butadiene by the rare earth catalysts was completely selective for alkene formation, and the yield of 2-butene was relatively high (>80%) with a high trans:cis ratio (2 ~ 20). The mode of hydrogen addition to the diene was examined using isotope techniques, indicating that 1-butene and 2-butene were formed by 1: 2- and 1:4-addition of hydrogen to 1,3-butadiene, respectively. In addition, it was found that the molecular identity of hydrogen was conserved during the hydrogenation of unsaturated hydrocarbons.