9-Borabicyclo[3.3.1]nonane (H-B-9-BBN) has been used as a catalyst for the sequential double hydroboration of alkynes with pinacolborane (HBpin) to give alkyl gem-di-pinacol boronic esters. This strategy, which is effective for a wide range of terminal alkynes, is predicated upon a key C(sp3)-B / B-H transborylation reaction. Transition-state thermodynamic parameters and 10-boron-isotopic labelling experiments are indicative of an σ-bond metathesis exchange pathway.