Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G** calculations of the C(5)H(5) potential energy surface have been performed to investigate the reaction mechanism of ethynyl radical (C(2)H) with C(3)H(4) isomers, allene and methylacetylene. They were followed by RRKM calculations of reaction rate constants and product branching ratios under single-collision conditions. The results show that the C(2)H + CH(2)CCH(2) reaction in a case of statistical behavior is expected to produce 1,4-pentadiyne (56-63%), ethynylallene (22-24%), and pentatetraene (10-15%), with the most favorable pathways including H losses from the initial HCCCH(2)CCH(2) adduct leading to either 1,4-pentadiyne or ethynylallene, and a multistep route HCCC(CH(2))(2) --> four-member ring --> CH(2)CCCHCH(2) --> CH(2)CCCCH(2) + H featuring a formal insertion of C(2)H into a double bond of allene followed by H elimination giving rise to pentatetraene. On the contrary, the C(2)H + CH(3)CCH reaction produces diacetylene + methyl (21-61%) by CH(3) loss from the HCCC(CH)CH(3) initial adduct as well as methyldiacetylene + H (27-56%) and ethynylallene + H (11-22%) by H eliminations from CHCCHCCH(3). The calculated product branching ratios are in general agreement with the available experimental data, although some quantitative deviations from experiment and possible reasons for them are also discussed. The present calculations confirm that the C(2)H + C(3)H(4) reactions proceed without entrance barriers and lead, via intermediates and transition states residing lower in energy than the initial reactants, to the C(5)H(4) + H and C(4)H(2) + CH(3) products exothermic by 20-36 kcal mol(-1), with strong dependence of the product distribution on the reacting C(3)H(4) isomer, making these reactions fast under low-temperature conditions of Titan's atmosphere where they can serve as a source of more complex unsaturated hydrocarbons.