The reactions of the ethynyl radical (C₂H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C₂H radical is prepared by 193-nm photolysis of CF₃CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron–photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C₅H₄ produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50–70% diacetylene (H–CC–CC–H) and 50–30% C₅H₄, with a C₅H₄-isomer distribution of 15–20% ethynylallene (CH₂CCH–CCH) and 85–80% methyldiacetylene (CH₃–CC–CCH). The reaction of allene with ethynyl gives 35–45% ethynylallene, 20–25% methyldiacetylene and 45–30% 1,4-pentadiyne (HCC–CH₂–CCH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan’s atmospheric chemistry are discussed.