The primary reactions of the lowest energy triplet states of diacetylene (C4H2*) with 1,3-butadiene (C4H6) in a helium buffer are characterized with a molecular beam pump-probe technique. Triplet diacetylene is prepared in the early portions of a molecular expansion by laser excitation of the 2(0)(1)6(0)(1) band of the (1) Delta(u) <-- X-1 Sigma(g)(+) transition in C4H2 at 231.5 nm, which rapidly interconverts to high vibrational levels of the lowest energy tripler surfaces. The subsequent reactions with C4H6 are allowed to proceed for 20 mu s while the expansion traverses a short ceramic reaction tube or slit channel. Primary products are observed by quenching secondary processes as molecular collisions cease outside the tube. The major photochemical products C6H6 and C8H6 are detected in a linear time-of-flight mass spectrometer using both vacuum ultraviolet photoionization and resonant two-photon ionization (R2PI). R2PI spectra of the C6H6 and C8H6 products unambiguously identify them as benzene and phenylacetylene, respectively. Based on deuterium substitution experiments, a mechanism for these ring-forming reactions is proposed. The potential importance of these reactions for forming aromatics in sooting flames and planetary atmospheres is discussed.