The rate coefficient for the reaction of the ethynyl radical (C2H) with 1-butyne (H-C C-CH2-CH3) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C2H2) at 193 nm and detected via chemiluminescence (C2H + O-2 -> CH (A(2)Delta) + CO2). The rate coefficients are measured over the temperature range of 74-295 K. The C2H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature-dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 +/- 0.5) x 10(-10)(T/295 K)(-(0.04+/-0.03)) cm(3) molecule(-1) s(-1). Reaction products are detected at room temperature (295 K) and 533 Pa using a multiplexed photoionization mass spectrometer (MPIMS) coupled to the tunable vacuum ultraviolet synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C5H4) and m/z = 78 (C6H6) corresponding to the CH3-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C5H4 products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C6H6 product channel includes two cyclic isomers, fulvene 18(+/-5)% and 3,4-dimethylenecyclobut-1-ene (DMCB) 32(+/-8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(+/-5)%, 3,4-hexadiene-1-yne 28(+/-8)%, and 1,3-hexadiyne 14(+/-5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C4H2) formation via the C2H5-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturn's largest moon Titan and the relationships to combustion reactions, are discussed.