The mechanism of R2BH-catalyzed hydroboration of alkynes by 1,3,2-dioxaborolanes has been investigated by in situ 19F NMR spectroscopy, kinetic simulation, isotope entrainment, single-turnover labelling (10B/2H), and density functional theory (DFT) calculations. For the Cy2BH catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the antiMarkovnikov addition product ArCH=CHBCy2. Irreversible and turnover-rate limiting reaction with pinacolborane (k ~7´10-3 M-1s1) regenerates Cy2BH and releases E-Ar-CH=CHBpin. Two irreversible events proceed in concert with turnover. The first is a Markovnikov hydroboration leading to regioisomeric E-Ar-CH(Bpin)=CH2. This is unreactive to pinacolborane at ambient temperature, resulting in catalyst inhibition every ~102 turnovers. The second is hydroboration of the alkenylboronate to give ArCH2CH(BCy2)Bpin, again leading to catalyst inhibition. 9-BBN behaves analogously to Cy2BH, but with higher anti-Markovnikov selectivity, a lower barrier to secondary hydroboration, and overall lower efficiency. The key process for turnover is B-H/C-B metathesis, proceeding by stereospecific transfer of the E-alkenyl group within a transient, µ -B-H-B bridged, 2-electron-3-centre bonded B-C-B intermediate.