A brief description is given of a "microsecond" flash photolysis apparatus in which a 40 µf. condenser charged to 8000 v. is discharged through a photolysis flash tube in ~20 microseconds. Absorption spectra of transient species are photographed with a second flash tube which provides a source of continuum by discharging a 2 µf. condenser charged to 10,000 v., in 3–5 microseconds. A circuit for controlling the time interval between the two flashes is given.Experiments on the flash photolysis of diacetylene are discussed. With diacetylene at 0.5 mm. Hg pressure several well-known band systems were photographed in absorption 20 microseconds after the beginning of the photolysis flash, viz., the C2 Swan bands, the C2 Phillips bands, the C2 Deslandres–d'Azambuja bands, the 4050 Å bands of C3, and the CH band at 3143 Å. The rotational temperature of these bands was ~3000°–5000° K. The C2 Swan bands were also recorded in emission after a single photolysis flash. When a considerable excess (100:1) of helium was added to the diacetylene, all the above band systems disappeared with the exception of the C3 bands.In the absence of helium it is probable that the reaction is mainly thermal and that the high temperature is produced by a thermal explosion of the diacetylene triggered by the photolysis flash. The thermal reaction is suppressed by the addition of excess helium and the photochemical reaction becomes dominant. Under these conditions it is interesting to note that C3, but no C2, is produced. It appears therefore that C3 is a product of the photochemical decomposition of diacetylene. Possible mechanisms are discussed.