The chemistry of the triplet metastable state of diacetylene (C4H2*) With ethene, propene, and propyne in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. An ultraviolet photoexcitation laser counterpropagates the molecular expansion through a short reaction tube, exciting the C4H2 (1) Delta(u) <-- (1) Sigma(g)(+) 2(0)(1)6(0)(1) and 6(0)(1) transitions at 231.5 and 243.1 nm, respectively. Efficient intersystem crossing forms the metstable triplet state from which reaction occurs. The short length of the tube (8 mm) serves to quench the reaction after 10-30 mu s so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuum ultraviolet light and mass-analyzed in a linear time-of-flight mass spectrometer. The primary products in the reactions with ethene (C6H4, C6H5), propene (C5H6, C5H6, C6H4, and C7H6), and propyne (C5H3, C5H4, C6H2, and C7H4) are consistent with poly-yne, enyne, and cumulene products. Percent product yields are determined assuming equal photoionization cross sections for the products. Relative photoionization cross sections at 118 mm for a series of model alkene, alkyne, enyne, diene, and diyne compounds are determined to test the variations in photoionization cross section expected for the products. Relative rate constants for the reactions (scaled to k(C4H2* + C4H2) = 1.00) with ethene, propene, and propyne are extracted from concentration studies, determining values of 0.24 +/- 0.01, 0.32 +/- 0.01, and 0.42 +/- 0.02 in helium buffer, respectively. Isotopic studies employing deuterated reactants are used to constrain the mechanisms for the reactions. Most of the major products are proposed to follow formation of an unbranched or branched chain adduct which subsequently decomposes by loss of interior atoms to form a stable poly-yne or en-yne product. Two schemes are proposed to account for formation of the isotopically labeled C5H4 and C5H3 products in the C4H2* + CH3C2H reaction. Only one of these mechanisms appears to be operative in the C4H2* + CH3CH=CH2 reaction.