We demonstrate that [Rh(η⁶-FC₆H₅)(Ph₂PCH₂CH₂CH₂PPh₂)][BAr^F₄] is a competent catalyst for the dehydrocoupling of H₃B·PR₂H to give H₃B·PR₂BH₂·PR₂H (R = Ph, ^tBu). The isolation of intermediates (R = ^tBu, Ph) and kinetic/isotopic labelling experiments (for R = Ph) point to a mechanism, for R = Ph, in which the rate-limiting process for dehydrocoupling involves B–H activation, while the turnover limiting process for catalysis involves the substitution of the oligomeric product at the metal, which forms a strong chelate complex with a Rh(III) centre, by H₃B·PPh₂H. When R = ^tBu no chelate product complex forms, and it is speculated that the dehydrocoupling process now becomes turnover limiting. These observations provide, for the first time we believe, data on the P–B bond forming events that are central to the dehydrocoupling and dehydropolymerisation of phosphine–boranes.