Laser-ablated boron atoms have been reacted with ethylene during condensation in excess argon. The BC2H3 closed-shell isomers borirene (CH)(2)BH, ethynylborane H2BCCH, and borallene HBCCH2 have been identified from infrared spectra by using B-10, C-13, and D substitution and BP86/6-311G* isotopic frequency calculations. The observation of both cyclic aromatic and aliphatic products suggests that the reaction proceeds through both exothermic C=C addition and C-H insertion reaction mechanisms. Evidence is presented for the most stable BC2H4 radicals, namely, cyclic BHCHCH2 and aliphatic H2BCCH2, which require H-atom migration within the intermediates first formed. Ethane gave a smaller yield of the same products, including linear HBCC and BCCH, vinylborane H2BC2H3, and an enhanced yield of the C-C insertion product CH3B=CH2.