Irradiation of aqueous solutions of biacetyl (2, 3–butanedione), oxygen and a variety of different azo dyes with light has been shown to result in the rapid oxidation of the dye to phenolic and quinonoid products. The quantum efficiency values of the dyes and the oxygen consumption during fading of some commercially available and some laboratory–synthesised azo dyes have been measured. The photo–oxidation was found to be inhibited by phenolic compounds. Since these compounds are products of the photo–oxidation of azo dyes, they could play an important role in the determination of the dyes' light fastness properties. As keto groups are present in many substrates, the ketone–sensitised photo–oxidation could be relevant in the fading of azo dyes.