The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 Pm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding. (C) 2009 Elsevier B.V. All rights reserved.