Photo-modulated voltammetry was used to det. the redox potentials of the diiodide radical (I2-·) in H2O, MeCN, and 3-methoxypropionitrile. Iodide/triiodide redox electrolytes were exposed to modulated blue light, resulting in I2-· generation. Using transparent FTO electrodes, 2 modulated photocurrent waves could be discerned in the voltammogram, from which the formal potentials for oxidn. and redn. reactions of the diiodide radical were detd. E0/(I2-·/I-) is +0.79 and +1.04 V vs. normal H electrode in MeCN and H2O, resp. These values give guidelines for E0/(D+/D) required for efficient regeneration of dyes used in dye-sensitized solar cells.