In this work, using an in-house made Loschmidt diffusion cell, we measure the effective coefficient of dry gas (O-2-N-2) diffusion in cathode catalyst layers of PEM fuel cells at 25 degrees C and 1 atmosphere. The thicknesses of the catalyst layers under investigation are from 6 to 29 mu m. Each catalyst layer is deposited on an Al2O3 membrane substrate by an automated spray coater. Diffusion signal processing procedure is developed to deduce the effective diffusion coefficient, which is found to be (1.47 +/- 0.05) x 10(-7) m(2) s(-1) for the catalyst layers. Porosity and pore size distribution of the catalyst layers are also measured using Hg porosimetry. The diffusion resistance of the interface between the catalyst layer and the substrate is found to be negligible. The experimental results show that the O-2-N-2 diffusion in the catalyst layers is dominated by the Knudsen effect. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.