In order to simulate the effects of high-temperature operating conditions on the through-plane gas permeability of gas diffusion layers (GDLs) used in polymer electrolyte fuel cells, uncoated and coated GDLs were heated at various temperatures (ie 200, 500 and 800 C). The results show that the through-plane gas permeability of the uncoated GDLs generally increases after higher temperature treatment. However, the coated GDL displays a different trend: the through-plane gas permeability increases with increasing temperature treatment to 200 and 500 C, but then decreases after heat treatment at 800 and 1000 C. With the assistance of SEM images, the above results are discussed.