The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The only primary photofragments observed were HCCH (acetylene) and C6H4 Some of the C6H4 molecules were found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of DeltaH(f) less than or equal to 160 +/- 4 kcal mol(-1) wad determined for 1,3,5-hexatriyne from the energetic threshold for this process, This experimentally determined value agrees well with our ab initio calculations performed at the G2 level of theory. Angular distribution measurements for the HCCH + C6H4 channel yielded an isotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded no evidence for the phenyl + ethynyl, dr the atomic hydrogen elimination channels even though these:were observed in the pyrolytic studies of phenylacetylene [Hofmann,J.; Zimmermann, G.; Guthier; K.; Hebgen, P.; Homann, K. ii. Liebigs Ann. 1995, 631, 1995, Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zimmermann, G. Liebigs Ann. 1995, 637, 1995].