A modified Loschmidt cell was used to measure the relative gas diffusivity (D*) of the porous catalyst layers (CLs) of polymer electrolyte membrane fuel cells as a function of CL ionomer/carbon weight ratio (I/C) (0.5, 1.1, and 1.5) and operating temperature (20, 40, and 72 degrees C). D* decreased by 80% when I/C was increased from 0.5 to 1.5. While the effective gas diffusivity of CL increased with temperature, D* decreased because binary diffusion increases more rapidly than Knudsen diffusivity with temperature. The structure of CL was modeled through considering a packed-sphere model for carbon particles within agglomerates, and a network of overlapped spherical agglomerates forming the CL. The gas diffusion problem was solved analytically for the CL structure considering both Knudsen and molecular mechanisms, and, results were validated. Using the model, the effect of porosity, pore size distribution and ionomer coverage on gas diffusivity was evaluated. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.