The distribution of water and water flooding inside the cathode gas channels has been successfully visualized by using a transparent protonexchange membrane fuel cell. The effect of various stoichiometries of cathode gas concentration and humidification are evaluated in a systematic way. The output voltage is controlled in the mass-transfer region (0.4-0.6V) and the cell temperatureis fixed at 50 degrees C. The cell performance increases with increasing stoichiometry of humidified oxygen until the stoichiometry reaches a critical value. On the other hand, the performance decreases with increasing stoichiometry of non-humidified oxygen. On replacing oxygen with humidified air, the cell performance and stoichiometry of the humidified air are in direct proportion due to the increasing mass-transfer rate. (c) 2006 Elsevier B.V. All rights reserved.