Degradation of N719 dye on TiO2 electrodes in dye-sensitive solar cells with high temperatures up to 80 degrees C and a visible light exposure were investigated using infrared absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) technique. In this study, we developed an in situ observation technique to measure structural changes in the N719-dye molecule on the TiO2 electrode even with a liquid iodine electrolyte. MIR-IRAS clearly indicates the structural changes in the N719 dye molecule such as degradation of NCS groups and hydrocarbon. Water generation was also detected on the TiO2 surface. Guanidine isothiocyanate addition into the electrolyte prohibits the degradation of NCS with the light exposure and the high temperature treatment. The degradations of dye, electrolyte, and TiO2 electrode in the dye sensitized solar cells with the light exposure and thermal treatments are discussed in this paper. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.053211jes] All rights reserved.