The crossed molecular beam reaction of boron monoxide (11BO; X2Σ+) with dimethylacetylene (CH 3CCCH3; X1A1g) was investigated at a collision energy of 23.9 ± 1.5 kJ mol-1. The scattering dynamics were suggested to be indirect (complex forming reaction) and were initiated by the addition of 11BO(X2Σ+) with the radical center located at the boron atom to the π electron density at the acetylenic carbon-carbon triple bond without entrance barrier leading to cis-trans11BOC4H6 doublet radical intermediates. cis-11BOC4H6 underwent cis-trans isomerization followed by unimolecular decomposition via a methyl group (CH3) loss forming 1-propynyl boron monoxide (CH3CC 11BO) in an overall exoergic reaction (experimental: -91 ± 22 kJ mol-1; theoretical: -105 ± 9 kJ mol-1; NIST: -104 ± 12 kJ mol-1) via a tight exit transition state; trans-11BOC4H6 was found to lose a methyl group instantaneously. Neither atomic nor molecular hydrogen loss pathways were detectable. The experimental finding of an exclusive methyl loss pathway gains full support from our computational study predicting a methyl group versus atomic hydrogen loss branching ratio of 99.99% to 0.01% forming 1-propynyl boron monoxide (CH3CC11BO) and 1-methyl-propadienyl boron monoxide (CH3(11BO)CCCH2), respectively. © 2014 the Owner Societies.