An apparatus for effecting flash photolysis in the vacuum ultra-violet region is described. Appreciable decompositions of water, carbon dioxide, acetylene, ethylene and methane were effected with a 2000J flash of 30 $\mu$s duration. A detailed study of water vapour photolysis has established that, in the primary step, at least 90% of the dissociation leads to a hydrogen atom and a hydroxyl radical. The kinetics of the hydroxyl radical disappearance, following flash photolysis of water vapour in the presence of various third bodies, has been studied. The rate constant is faster in xenon than in helium and other similarities with the data on iodine atom recombination suggest the operation of a mechanism involving intermediate complex formation.