We studied the mechanism of the degradation of I-/I(3)(-)containing electrolyte solutions for dye-sensitized solar cells under UV light irradiation. The yellow electrolyte solutions underwent achromatization during irradiation, indicating the reduction of I-3(-). We propose a mechanism involving the production of holes in TiO2, reaction of the holes with solvent molecules, and subsequent reduction of I-3(-) by electrons remaining in the TiO2. Although the quantum yield of the photodegradation reaction is estimated to be low (3 x 10(-3)), this reaction can nevertheless be expected to affect the long-term stability of dye-sensitized solar cell devices. (C) 2014 Elsevier B.V. All rights reserved.