The reversible partitioning of molecules between immiscible phases, such as in solvent extraction, is usually controlled by chemical composition of one or both phases, temperature, or pH. An additional means for controlling the equilibrium distribution of a solute between phases, the use of light and suitable photochromic molecules, such as those drawn from the spiropyran family, is described. Partitioning of 1',3',3'-trimethyl-6-nitrospiro[2H-1]-benzopyran-2,2' indoline (1'-methyl 6-NO2 BIPS) and its derivatives between toluene and water phases is shown to be controlled by the wavelength of incident light, the ionogenic functional groups on the molecule, and the aqueous solution pH. At pH 2, both 1'-methyl 6-NO2 BIPS and 1'-(3-carbomethoxypropyl) 6-NO2 BIPS partition reversibly and preferentially into the aqueous phase when irradiated with ultraviolet light in the 365-370 nm region but only slightly into the aqueous phase when illuminated with wavelengths greater than or equal to 480 nm. The partition coefficient of these spiropyrans at pH 2 and under irradiation with ultraviolet light is 25 times larger than when irradiated with visible light. For aqueous solutions of 1'-(3-carboxypropyl) 6-NO2 BIPS, the partition coefficient at pH 4 and under UV light irradiation is 22 times larger than that under visible light irradiation, whereas at pH 6 the partition coefficient under UV irradiation is only 2 times greater than under visible light irradiation. These partition coefficient variations are explained by applying chemical equilibrium theory to the reversible, irradiation-induced structural changes in spiropyran molecules. The fit of theory to the data confirms the importance of interface speciation.