The reaction dynamics of boron monoxide (11BO; X2Σ+) with propylene (CH3CHCH2; X1A′) were investigated under single collision conditions at a collision energy of 22.5 ± 1.3 kJ mol-1. The crossed molecular beam investigation combined with ab initio electronic structure and statistical (RRKM) calculations reveals that the reaction follows indirect scattering dynamics and proceeds via the barrierless addition of boron monoxide radical with its radical center located at the boron atom. This addition takes place to either the terminal carbon atom (C1) and/or the central carbon atom (C2) of propylene reactant forming 11BOC3H6 intermediate(s). The long-lived 11BOC3H6 doublet intermediate(s) underwent unimolecular decomposition involving at least three competing reaction mechanisms via an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group elimination to form cis-/trans-1-propenyl-oxo-borane (CH3CHCH11BO), 3-propenyl-oxo-borane (CH2CHCH211BO), and ethenyl-oxo-borane (CH2CH11BO), respectively. Utilizing partially deuterated propylene (CD3CHCH2 and CH3CDCD2), we reveal that the loss of a vinyl hydrogen atom is the dominant hydrogen elimination pathway (85 ± 10%) forming cis-/trans-1-propenyl-oxo-borane, compared to the loss of a methyl hydrogen atom (15 ± 10%) leading to 3-propenyl-oxo-borane. The branching ratios for an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group loss are experimentally derived to be 26 ± 8%:5 ± 3%:69 ± 15%, respectively; these data correlate nicely with the branching ratios calculated via RRKM theory of 19%:5%:75%, respectively. © 2014 American Chemical Society.