The photolysis of pyruvic acid vapour has been studied at wavelengths of 366, 345, and 320 nm, at a temperature of 340 K and pressures from about 1 to 10 Torr. Products observed were CO2 and CH3CHO, with the former always in excess, and a quantum yield of CO2 of 0.9 ± 0.1 at 366 nm. Light emission was also observed, and from the effect of added O2 it was concluded that emission occurred from both singlet and triplet states. It is suggested that the primary process in the photolysis is an internal hydrogen-atom transfer followed by dissociation into CO2 and CH3—C—OH, with the latter then rearranging to give CH3CHO and other products.The absorption spectrum is reported; the first system begins at about 380 nm, rising to a maximum at about 350 nm, with εmax = 10 M−1 cm−1, and is attributed to the first excited π* ← n+ singlet state. Weak absorption below 300 nm is probably due to the π* ← n− state and much stronger absorption below 220 nm to the π* ← π state.The thermal decomposition was studied briefly at temperatures from 455 to 584 K and pressures from about 0.8 to 7 Torr. Products were the same as in the photolysis, and Arrhenius parameters of log A (s−1) = 7.19 and E = 27.7 kcal/mol were obtained, based on the formation of CO2, which was apparently a simple, homogeneous unimolecular process.