A thorough understanding of the oxygen transport properties in the gas diffusion layers (GDLs) of polymer electrolyte membrane (PEM) fuel cells is crucial for improving cell performance. However, because of the complex pore structure of the GDLs and the presence of water in them, the effective oxygen diffusivity (EOD) in porous medium of a PEM fuel cell is still not precisely understood. In this paper, the EODs were measured in two typical porous materials (TGP-H-120 carbon paper and carbon cloth, commonly used as GDLs) by using an oxygen sensor based on a galvanic cell under both dry and partially saturated conditions. The measurements were performed for various pore size distributions (PSDs) and water saturation distributions (WSDs); different PSDs were obtained by increasing the PTFE loading and different WSDs were realized via two impregnation methods using liquid water, namely vacuum impregnation and moist air condensation impregnation. The EOD was found to strongly depend on the PSD and WSD in the GDL, not simply on the average porosity and average water saturation, as reported in previous studies. Finally, this paper presents the experimentally verified equations that account for PSDs and WSDs under both dry and partially saturated conditions for the prediction of the EODs in PEM fuel cells. (C) 2016 Elsevier Ltd. All rights reserved.