The collisional photochemistry of diacetylene triplet metastable state (C4H2*) with acetylene in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. The photolysis laser counterpropagates the molecular expansion through the reaction tube exciting the C4H2 'Au -I&+ 2'06'0 transition at 231.4 nm. Efficient intersystem crossing forms the triplet state from which reaction occurs. The finite 8 mm length of the tube serves to quench the reaction after 10-30 ps so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuumultraviolet light and mass analyzed in a linear time-of-flight mass spectrometer. The primary photochemical processes are C4H2* + C2Hz -C6H2 + 2H (H2) and C4H2* + C2H2 -C6H3 -I-H. Under all conditions examined, C6H2 dominates the observed products, with C6H3 present above the I3CCsH2 background only at high nitrogen concentrations which deactivate diacetylene within its metastable vibrational manifold more efficiently than does helium. C6DH is the major product in the C4H2* + C2D2 reaction, indicating that both C4H2* and C2H2 contribute one hydrogen to the C6H2 product. CS& is observed as a new minor product of the reaction of excited diacetylene with C a 2 , but only at long reaction times. Formation of c 8 H 4 is significantly enhanced by the presence of C2H2. A simple mechanism for the reaction of diacetylene and acetylene with metastable excited diacetylene is formulated. Assuming equal photoionization cross sections for the products, concentration data are used to determine that k(C4H~)/k(C2H2) = 11 f 2 in helium buffer and 7 f 1 in nitrogen buffer, where k(i) is the total rate constant for the reaction of C4H2* with species i.