A counter-now type 40-cell proton exchange membrane fuel cell (PEMFC) stack with an effective electrode area of 200 cm 2 has been assembled and its performance was investigated. Under the condition of atmospheric pressure and 75 degreesC, the maximum power of the stack was 2.89 kW (0.36 W/cm(2) per cell) and 2.3 kW (0.29 W/cm(2) per cell) for H-2/O-2 and H-2/Air, respectively. However, the stack showed a rapid decay in performance after continuous operation of 1800 h. Electron probe micro analyzer (EPMA) and X-ray fluorescence (XRF) spectrometer analyses revealed that degradation of the catalyst and contamination of the polymer electrolyte membrane were the main causes of such sudden decay.