Electrochemical impedance spectroscopy (EIS) was employed for in situ diagnosis for polymer electrolyte membrane fuel cells during operation. First, EIS was measured as a function of operation parameters such as applied current density, gas flow rates and gas humidification temperature. The resistance that correlated with conductivity of the membrane and the contact resistance between bipolar plate and gas diffusion layer (GDL) was set as R-m in the assumed equivalent circuit. The charge transfer resistances were considered for cathode (R-ct(C)). The value of R-ct(C) was sensitive to the parameters that affected cell voltage. Additionally, the diffusion resistance (R-d) was ascribed to the effect of oxygen supply and drainage of generated water. Second. the influence of corrosion of type 430 stainless steel bipolar plates was evaluated by EIS method during operation. Corrosion of the stainless steel bipolar plates resulted in an increase in the value of Rd. (C) 2010 Elsevier B.V. All rights reserved.