Reduced production costs and enhanced durability are necessary for practical application of polymer electrolyte fuel cells. There has been a great deal of concern about degradation of the gas diffusion layer located outside the membrane electrode assembly. However, very few studies have been carried out on the degradation process, and no suitable methods for improving the durability of the cell have been found. In this work, the influence on the cell performance and factors involved in the degradation of the gas diffusion layer has been clarified through power generation tests. Long-term power generation tests on single cells for 6000 h were carried out under high humidity conditions with homogeneous and inhomogeneous hydrophobic coating gas diffusion layers. The results showed that the increase in the diffusion overvoltage from the gas diffusion layer could be controlled by the use of a homogeneous coating. Post-analyses indicated that this occurred by controlling oxidation of the carbon fiber. (C) 2011 Elsevier B.V. All rights reserved.