Reactions of laser-ablated thorium and uranium atoms with ethylene isotopomers have been carried Out and the primary products identified in the matrix IR spectra. The dihydrido cyclic and trihydrido ethynyl products (MH2-C2H2 and MH3CCH) are identified from the matrix spectra of both Th and U, whereas the insertion product absorption is observed only in the U spectra. The present results indicate that C-H insertion and following H migration from C to M also occur in reaction of the actinides with ethylene. Formation of the trihydrido ethynyl products is parallel to the previous results of group 4 metals. Metal hydride (MHx) absorptions are not observed in the infrared spectra, suggesting that the H-2 elimination from the dihydrido cyclic intermediate is relatively slow. Density functional theory calculations reproduce the vibrational characteristics of the identified products and the relative stabilities. The cyclic triangular ThH2-C2H2 system with two pi electrons is aromatic, which contributes to the unique stability of this thorium dihydrido cyclic product.