The reaction dynamics of ground state boron atoms, B(P-2(j)), with acetylene, was reinvestigated and combined with novel electronic structure calculations. Our study suggests that the boron atom adds to the carbon-carbon triple bond of the acetylene molecule to yield initially a cyclic intermediate undergoing two successive hydrogen atom migrations to form ultimately an intermediate i3. The latter was found to decompose predominantly to the c-BC2H(X(2)A') isomer plus atomic hydrogen via a tight exit transition state. To a minor amount, an isomerization of i3-i4 prior to a hydrogen atom ejection forming the linear structure, HBCC(X-1 Sigma(+)), has to be taken into account. Since the c-BC2H(X(2)A') and HBCC(X-1 Sigma(+)) isomers are separated by an isomerization barrier to ring closure of only 3 kJ mol(-1), internally excited HBCC(X-1 Sigma(+)) products can isomerize to the c-BC2H(X(2)A') structure and vice versa. (C) 2007 Elsevier B.V. All rights reserved.