Ab initio G3(MP2, CC)//B3LYP/6-311G** calculations have been performed to investigate the C2H + C6H6 and C4H3 + C4H4 reactions on the C8H7 potential energy surface. The results demonstrate that C2H reacts with benzene without a barrier and then the C6H6(C2H) adduct produced loses atomic hydrogen to form phenylacetylene with overall reaction exothermicity of 28.4 kcal/mol. The reaction can be a major source of phenylacetylene under low-temperature conditions of Titan's atmosphere. The reactions of vinylacetylene and butatriene with i-C4H3, producing phenylacetylene, pentalene, or benzocyclobutene, are predicted to be unlikely at low temperatures because of significant barriers but may be important in combustion flames. (C) 2008 Elsevier B.V. All rights reserved.