The ultraviolet photolysis of dilute solutions of diphenylacetylene in hexane containing dissolved oxygen has been investigated. The main product of the reaction is benzoic acid. Benzene, hexylhydroperoxide, and 2- and 3- hexyl benzoates are also formed. The reactive intermediate is thought to be the benzoyloxy-radical formed by the addition of a molecule of oxygen to a molecule of diphenylacetylene in its triplet excited state. Photoaddition of a further molecule of oxygen to this intermediate and subsequent breakdown of the resulting molecule forms two benzoyloxy-radicals. The observed products may be formed from the benzoyloxy-radicals by abstraction of hydrogen from the solvent or by elimination of carbon dioxide followed by hydrogen abstraction by the resulting phenyl radicals. The effect of photosensitisers, quenchers, and a change of solvent on the reaction have been studied.