The goal of this study is to evaluate a reaction mechanism which leads to the formation of a closo-carborane starting from B4H10 + C2H2. Experimentally, it is known that alkylboranes, nido-carboranes, and closo-carboranes can all be obtained depending on reaction conditions. For this study, geometries were computed at the MP2/6-31G(d) level and relative energies were estimated at the [MP4/6-311+G(d,p)] level. The first assumed step is the elimination of BH3 from B4H10 to form B3H7. Rather than the expected hydroboration product, the initial reaction of B3H7 + C2H2 gives an addition product with little or no barrier. Loss of Ha leads first to cyclic-C2B3H7 carboranes and then through two methyleneborane intermediates to the known nido-C2B3H7. Two pathways exist for loss of H-2 from nido-C2B3H7, one synchronous and the other nonsynchronous, to the final product, 1,2-C2B3H5. With respect to B4H10 + C2H2, four activation barriers exist in the range 45-50 kcal/mol.