The through-plane permeability has been experimentally measured for microporous layer (MPL)-coated gas diffusion layers (GDLs) as used in proton exchange membrane fuel cells (PEMFCs). Provided that the carbon substrate is maintained unchanged, the results show that the through-plane permeability of the tested GDLs increases with an increase in the quantity of PTFE in the MPL. The analysis that is undertaken to investigate the effect of the air compressibility on the through-plane permeability shows that there is an underestimation in the permeability by up to 11% if the density of the air is assumed to be constant. Neglecting the compressibility of the air does not have the same effect on the inertial coefficients of the tested GDLs as it is overestimated, or underestimated, depending upon the tested GDL. Also, a non-dimensionlisation analysis shows that the exclusion of the non-Darcy terms results in significant errors, especially at the reported maximum flow rates. Finally, the permeability of the MPLs was estimated and found to be 2-3 orders of magnitude lower than that of the corresponding carbon substrates. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.