The catalytic hydrodechlorination of BCl3 with molecular hydrogen in the presence of tertiary amines is a viable strategy for the energy-efficient generation of valuable B–H bonds. A mechanistic study based on experiments with isolated intermediates and deuterium labeling experiments is presented. The occurrence of the rate-limiting reverse reaction from the insoluble Et3NHCl adduct was identified as a major cause of low Et3NBH3 yields. In addition, amines with NCH2 units, which also serve in the corresponding cases as solvents, have a strong influence on the reaction kinetics; they are directly involved in the hydrogen transfer at elevated temperatures and assume the role of a cocatalyst. A catalytic cycle for the reaction on a nickel boride catalyst is proposed.