Transition structures and reaction paths for the BHmCln system have been characterized at the MP2(full)/6-31G(d) level of theory; heats of reaction and barriers have been computed at the G2 level of theory. Calculations show that the insertion reactions of BHmCln into H-2 and HCl (m + n = 0, 1, 2) occur by highly distorted, non-least motion transition states, with barriers that increase with chlorine substitution on boron. Hydrogen abstraction from BH and BH3-nCln by chlorine proceeds with little or no barrier by a linear transition state. By contrast, BH3-nCln + H --> BH2-nCln + H-2 occurs by an addition-elimination path via a tetracoordinate intermediate. Likewise, BH2-nCln + X --> BH1-nCln + HX or BH2-nCln-1 + XCl (X = H, Cl) are also addition-elimination processes. Abstraction of Cl from BCl and BH3-nCln by H is endothermic and occurs by a bent transition state.