The mechanism of R2BH-catalyzed hydroboration of alkynes by 1,3,2-dioxaborolanes has been investigated by in situ F-19 NMR spectroscopy, kinetic simulation, isotope entrainment, single-turnover labeling (B-10/H-2), and density functional theory (DFT) calculations. For the Cy2BH-catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the anti-Markovnikov addition product ArCH = CHBCy2. Irreversible and turnover-rate limiting reaction with pinacolborane (k approximate to 7 x 10(-3) M-1 s(-1)) 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 Ar-C(Bpin)=CH2. This is unreactive to pinacolborane at ambient temperature, resulting in catalyst inhibition every similar to 10(2) 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, mu-B-H-B bridged, 2-electron-3-center bonded B-C-B intermediate.