The basic performance of a banded structure membrane fuel cell module (rated power of ), which consisted of 15 polymer electrolyte membrane fuel cells laid out in a plane, was experimentally investigated. The results show that the module operated for a much longer time at an inclination angle, , of than at or , where it experienced a sudden power breakdown at the rated operating point. The output voltage and internal impedance of each cell in the module were specifically monitored over a long-term operation. Measurements were made of the temperature distribution of the entire module as well as of the oxygen concentration and relative humidity at a specific cell. Airflow near the cathode in a single cell was also visualized. It was concluded that the power breakdown was probably caused by flooding in the anode of one of the most downstream cells of the module.