Degradation sequence and electron kinetics in an aging dye-sensitized solar cell (DSSC) is proposed based on the results of electrochemical impedance spectroscopy (EIS). The degradation begins with desorption of excess dye which leads to efficiency improvement and an increase in both the open-circuit voltage (V(OC)) and charge transfer resistance (R(CT)). Subsequent degradation mechanisms leading to steady reduction in the efficiency include infiltration of electrolyte into the mesoporous TiO(2) film and aggregation of dye. The former leads to more active recapturing of injected electron by the redox couple and is associated with a reduction of R(CT) due to higher electron exchange current. Dye aggregation is proposed to be the main degradation mechanism at the end of the aging period. This is substantiated on the increase in R(CT) along with the decline in the chemical capacitance of TiO(2), C(mu). (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.009112jes] All rights reserved.