We investigated the formation of a high-viscosity wormlike-micelle solution consisting of a mixed aqueous solution containing the cationic surfactant cetyltrimethylammonium bromide (CTAB) and sodium cinnamate (NaCin) and control of the solution viscosity by photoisomerization of the NaCin. The aqueous CTAB/NaCin solution exhibited higher zero-shear viscosity than a classic wormlike micelle-forming system CTAB/sodium salicylate of the same concentration. UV irradiation of the aqueous CTAB/NaCin wormlike-micelle solution resulted in a remarkable decrease in its viscosity, from 6.6 x 10 to 2.1 x 10(-3) Pa s.