An ab initio potential-energy surface for the ground doublet state of BH+3 has been constructed at the QCISD(T)/6-311G(d,p) level of theory. Classical simulations of the collision between BH+ and H2(D2) and isotopic analogs show that the deep BH+3 well gives rise to substantial “scrambling” of the hydrogen and deuterium atoms leading to BHD++D, BD++HD, and BD+2+H products.