Voltage decreases of polymer electrolyte fuel cells (PEFC) are affected by the relative humidity of the reaction gas inside the cells. A study was conducted to establish a method for measuring relative humidity and current distribution inside PEFC cells in order to identify the factors affecting the voltage decay of such fuel cells. The humidity distribution was measured using a humidity sensor for directly monitoring the relative humidity of the reaction gas flowing through the air flow channel of the cathode separator. The current distribution was measured directly by attaching a current sensor to the rib of the cathode separator. Typical results of relative humidity and current distribution are described and interpretations are discussed. (c) 2005 Elsevier B.V. All rights reserved.