For polymer electrolyte membrane (PEM) fuel cells, the pore structure and small effective diffusion coefficient (EDC) of the catalyst layers have significant impact on the cell performance. In this study, both the pore structure and EDC of the catalyst layers are investigated experimentally; the pore structure of the catalyst layer is characterized by the method of standard porosimetry, and the EDC is measured by a modified Loschmidt cell for oxygen-nitrogen mixture through the catalyzed electrodes. It is found that Pt loading has a direct impact on the pore structure and consequently the EDC of the catalyzed electrodes. As the Pt loading is increased, the porosity and mean pore size of the catalyzed electrode decrease, and the EDC decreases accordingly, however, it is increased by 15-25% by increasing the temperature from 25 degrees C to 75 degrees C. The EDC of the catalyst layer is about 4.6 x 10(-7) m(2) s(-2) at 75 degrees C, compared with 25.0 x 10(-7) m(2) s(-1) for the uncatalyzed electrode at the same temperature. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.