Long-term durability and degradation mechanism of dye-sensitized solar cells (DSC) sensitized with organic dyes was studied with a yellow indoline dye (D131). Durability of DSC using D131 depended on electrolyte composition. The degradation mechanism of D131 were studied by model reaction in the organic solvent. Major degradation product was identified as decarboxylated D131 (DC131), and it was found that degradation was promoted by iodine and/or amine in the electrolyte. Analysis of degradation product in actual degraded cells showed that DC131 was produced in the cell. DSC with optimized electrolyte exhibited good outdoor stability over several months. (C) 2009 Elsevier B.V. All rights reserved.