The reaction between the ethynyl radical, C2H (X (2)Sigma (+)), and methylacetylene (X (1)A(1)'), which yields ethynylallene, pentadiyne, and butadiyne, has been studied at the density functional (B3LYP/6-311+G**) and coupled cluster (coupled-cluster single double perturbative triple/cc-pVTZ) levels of theory. These results agree with data from crossed molecular beam experiments where ethynylallene (10) and pentadiyne (13) have been observed. The C2H(1) radical initially attacks the pi system of methylacetylene (2) without an entrance barrier to form Z-1-ethynylpropen-2-yl (3) or Z-2-ethynylpropen-1-yl (4) in highly exothermic reactions. Geometric considerations as well as the computed enthalpies suggest Z-1-ethynylpropen-2-yl (3) to be the dominant initial intermediate. Assuming single collision conditions as found in cold molecular clouds in the interstellar medium and distinct planetary atmospheres, numerous rearrangements may ensue the initial reaction step before ejection of a hydrogen atom or a methyl group releases the accumulated reaction energy. (C) 2001 American Institute of Physics.