Photochemical control of micellar solubilization of an oily substance was investigated using the photoisomerization of 4-butylazobenzene-4 '-(oxyethyl)trimethylammonium bromide (AZTMA), a cationic surfactant modified with azobenzene. Examination of the effect of ultraviolet and visible light irradiation on the UV/vis absorption spectrum of aqueous AZTMA, solution revealed that the surfactant undergoes reversible isomerization between the trans and cis forms. The critical micelle concentrations (cme) of the two isomers determined electroconductometrically were 2.7 mM for the trans form and 8.2 mM for the cis form, respectively. Ultraviolet irradiation of aqueous trans-AZTMA solution with solubilized ethylbenzene caused photoisomerization of the trans isomer to release a part of the solubilized ethylbenzene. Subsequent visible light irradiation of the aqueous cis-AZTMA solution produced photoisomerization to the trans isomer to resolubilize the released ethylbenzene. Such control by light of micellar solubilization of an oily substance was found to arise from the differences in both the number and solubilizing capacity of micelles between the two isomers.