In this study, a new electrical equivalent circuit is developed to evaluate the performance of polymer electrolyte fuel cell (PEFC) stacks using electrochemical impedance spectroscopy (EIS). Experimental EIS measurements were carried out in an open-cathode PEFC stack to validate the new electrical equivalent circuit. The electrical equivalent circuit developed in the authors' previous study, which simulates the impedance response of a single PEFC, is applied to EIS measurements carried out in the open-cathode PEFC stack. However, it cannot reproduce EIS measurements with positive imaginary components at low frequencies. Thus, in this study, the electrical equivalent circuit is modified by adding electrical components which represent intermediate adsorbed species in a two-step electrochemical reaction as reported in the literature. The results show that the new, electrical equivalent circuit can accurately reproduce the experimental EIS measurements and can give an insight into the factors that limit the performance of the PEFC stack. This new electrical equivalent circuit can enable an assessment of the state of health and performance of the fuel cell stacks. (C) 2013 The Electrochemical Society. All rights reserved.