The aim of this study is to propose a general understanding of the photodegradation mechanism of azo dyes in both azo and hydrazone forms on the basis of reactive oxygen quenching experiments. Results revealed that singlet oxygen and hydroxyl radicals were only observed during the photochemical reaction of azo dyes in the hydrazone form under UV irradiation rather than dyes in azo form. Hydroxyl radicals were the dominant reactive species involved in the photodegradation of hydrazone dyes. Furthermore, the photodegradation mechanisms for azo dye in its azo and hydrazone forms were proposed respectively.