Differently from water,effcient photochromism with a strong colour contrast has been observed for the multistate compound 7,4'- dihydroxyflavylium in the presence of cetyl trimethylammonium bromide ( CTAB) micelles. Two states are responsible for the photochromism: trans-chalcone ( inside the micelle) in the dark, and flavylium cation, AH(+), ( in bulk water) upon irradiation. The kinetics of the system was characterized by. ash photolysis and pH jumps. Evidence that the photochemical process leading to AH(+) occurs in the micelle was achieved. The best colour contrast is obtained at pH = 1.5, from a solution practically colourless in the dark, to an intense yellow upon irradiation ( quantum yield 0.4). The system is completely reversible with a lifetime of 38 min at room temperature, and exhibits a reasonable stability. A kinetic model capable of fitting the data from thermal entrance of the compound into the micelle, its ejection to bulk water upon irradiation and quantum yields of the photochemical process is proposed.