The doublet potential-energy surface for the reaction of C2H with O, including three minimum isomers and three transition states, is explored theoretically using the coupled cluster and density functional theory. The initial association between CH and 0 is confirmed to be a barrierless process forming a low-lying adduct named as 1 (HCCO), followed by C-C bond rupture leading to product P-1(CH + CO), which might be the most abundant considering form both energetic and entropic factors. Less competitively, 1 can lead to P-2(CCO + H) directly via C-H bond cleavage or undergo H-shift and ring-closure to 2(c-COC-H), and then take H-shift and ring-opening to 3(HOCC) followed by dissociation to P-2(CCO + H). Because the intermediates, transition states and products involved in the feasible pathways all lie below the reactants, the C2H + 0 reaction is expected to be rapid, as is confirmed by experiment. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for the modeling of ethynyl-oxygen combustion chemistry. (c) 2007 Elsevier B.V. All rights reserved.