This study investigated the degradation mechanism of Ti substrate-based dye-sensitized solar cells (DSSCs) after a thermal aging test. The deteriorated component of Ti-based DSSCs was clarified by chemical impedance spectroscopy and scanning electron microscope. This indicated that an unfavorable reaction occurred on the Pt counter electrode, leading to a decrease of the fill factor. The device components, that is, counter electrode and electrolyte, were separated from the cell to trace the degradation factor. The factors for counter electrode catalytic ability degradation were analyzed by cyclic voltammetry. These results indicate that I-2 and Li+ coupled with water led to an unfavorable reaction on Pt counter electrode, and that water content in the electrolyte may accelerate Pt degradation. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.111204jes] All rights reserved.