The mechanism of RBH-catalyzed hydroboration of alkynes by 1,3,2-dioxaborolanes has been investigated by in situ F NMR spectroscopy, kinetic simulation, isotope entrainment, single-turnover labeling (B/H), and density functional theory (DFT) calculations. For the CyBH-catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the anti-Markovnikov addition product ArCH = CHBCy. Irreversible and turnover-rate limiting reaction with pinacolborane ( ≈ 7 × 10 M s) regenerates CyBH and releases -Ar-CH═CHBpin. Two irreversible events proceed in concert with turnover. The first is a Markovnikov hydroboration leading to regioisomeric Ar-C(Bpin)═CH. This is unreactive to pinacolborane at ambient temperature, resulting in catalyst inhibition every ∼10 turnovers. The second is hydroboration of the alkenylboronate to give ArCHCH(BCy)Bpin, again leading to catalyst inhibition. 9-BBN behaves analogously to CyBH, 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 -alkenyl group within a transient, μ-B-H-B bridged, 2-electron-3-center bonded B-C-B intermediate.