Ab initio/RRKM calculations have been performed to investigate the mechanism of the C2H + 1,2-butadiene reaction and to compute its product branching ratios under single-collision conditions. The reaction starts with barrierless C2H addition to various sites of H2C=C=CHCH3 producing different exothermic initial adducts. The chemically activated C6H7 adducts can then isomerize and decompose by splitting H or CH3. With the assumption of equal formation probabilities of all initial adducts, C6H6 (2-ethynyl-1,3-butadiene) + H are predicted as the dominant products (91-84% at collision energies of 0-7 kcal/mol), whereas C5H4 (penta-1,4-diyne) + CH3 (7-12%) and ethynylallene + CH3 (2-3%) are minor products. (C) 2011 Elsevier B.V. All rights reserved.