Abstract Fading phenomena of azo oil dyes in aqueous solutions of mixed surfactant systems have been studied by spectrophotometry. The fading phenomenon is observed when 4-phenylazo-1-naphthol (4-OH) is added to an aqueous solution of a mixed surfactant system (anionic-nonionic), although it does not occur in single surfactant solutions or in cationic-nonionic mixed surfactant system solutions. The fading rate of 4-OH is dependent on the concentrations of the mixed surfactant system and dye and on the molar ratios of the surfactants. A typical system occurs at 1.0 × 10 −2 mole/liter of mixed surfactant, about 6.0 × 10 −5 mole/liter of dye, and 0.5 molar ratio of surfactant. No fading phenomenon is observed when naphthalene-1-azobenzene (NA) is added to aqueous solutions of the mixed surfactant system at any mole fraction. This may be because, when a tautomer of azo oil dye penetrates the palisade layers of micelles, the singlet oxygen due to the hydrophilic-hydrophilic interaction between anionic and nonionic surfactant molecules attacks the azo groups of the tautomer.