The characteristics of lanthanide catalysts obtained when Sm and Yb were vaporized into a frozen organic (tetrahydrofuran, benzene and methyl-cyclohexane) matrix (77 K) were investigated. These low-valent, highly dispersed lanthanide particles (indicated as Sm/THF, Sm/benzene, Yb/THF, Yb/benzene etc.) were catalytically active and selective for hydrogenation and isomerization. Samarium usually showed a greater activity than ytterbium. Olefin [ethene, propene, but-1-ene and (z)-but-2-ene] hydrogenation obeyed the rate law v=kP_H, suggesting that the reaction is controlled by catalytic activation of hydrogen. The molecular isotopic identity of hydrogen was conserved during the hydrogenation. Yb/THF and Yb/benzene were active for partial hydrogenation of benzene to cyclohexene. For the hydrogenation of olefins and acetylenes the substrate specificity was high; thus C—C double bonds were more readily reduced than triple bonds. The samarium and ytterbium catalysts discriminate between terminal and internal C—C triple bonds, only internal CC bonds (but-2-yne and pent-2-yne) being reduced very selectively in contrast to acetylene, methylacetylene and but-1-yne. Solid base character of the lanthanide provides a cause for these differences in catalytic properties.