The reaction of ground-state boron atoms, B (P-2(j)), with dimethylacetylene, CH3CCCH3 (X(1)A(1g)), was studied at a collision energy of 17.9 kJ mol(-1) using the crossed molecular beam approach. It was found that the reaction follows indirect scattering dynamics. B (P-2(j)) attacks the carbon-carbon triple bond of the dimethylacetylene molecule to form a cyclic BC4H6 intermediate that undergoes hydrogen transfer from the methyl group to the boron atom. This complex then rotates around the A/B axis and fragments to atomic hydrogen plus a cyclic BC4H5 isomer, 1,2-dimethylene-3-bora-cyclopropane, via a tight transition state.