The reactions of B+ + nH2 to produce BH2+(H2)(n-1) have been studied by high-level ab initio techniques. The reaction mechanism and associated activation energy is found to depend dramatically on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise: first breaking the H2 bond and forming one BH bond followed by forming the second BH bond. This process has an activation energy of about 57 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism though a planar cyclic transition state where two H2 bonds are broken while simultaneously two BH bonds and one new Ha bond are formed. The activation energy for this process decreases dramatically from the n = 1 value to only about 11 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism; however, the actual insertion occurs late in the reaction after over 75% of the exothermicity has been realized. The addition of the third H2 molecule decreases the activation energy to only about 3.4 kcal/mol. For n = 4, the reaction mechanism is essentially identical to that of the n = 3 case. However, the fourth H2 causes the activation energy to increase by about 2 kcal/mol relative to the n = 3 case because the additional H2 molecule causes one of the other three H2 molecules to be slightly further away from the boron ion in the transition state geometry. The computational results are compared with the experimental results of Kemper, Bushnell, Weis, and Bowers (J. Ant Chem. Sec. cluster ion is the most reactive. On the basis of a comparison of experimentally determined magnitude and isotopic dependence of the activation energies with the computed adiabatic reaction barriers, it is suggested that the observed reaction rate may be dominated by a nonclassical tunneling contribution.