Experiments examining the influences of gas diffusion layer (GDL) design parameters on the performance of polymer electrolyte fuel cells (PEFCs) are carried out under controlled clamp pressure conditions. The PEFC performance is strongly affected by the humidity of the supplied gas. Under moderate-humidity conditions, improved performance is obtained using an untreated GDL with high air permeability, whereas under high-humidity conditions, improved performance is obtained using a GDL treated with PTFE. The addition of a microporous layer (MPL) under humid conditions significantly reduces flooding on the electrode, thereby decreasing the concentration overpotential. The MPL is also effective for preventing the membrane electrode assembly (MEA) from drying out under low-humidity conditions.