Operating Polymer Electrolyte Fuel Cells (PEFC) under high current density conditions, causes significant losses related to liquid water saturation in the gas diffusion layer (GDL). The blockage of pores inside the material has a strong influence on its effective gas transport properties. Here we report on the combination of in-situ X-ray tomographic microscopy (XTM) of PEFC and the numerical determination of gas transport properties using Lattice Boltzmann and finite difference methods. The GDL domains (Toray TGP-H-060) of two identical cells, each with 11 mm(2) active area, were analyzed in sections of about 0.3 to 0.8 mm(2) size. Saturation levels between 0.1 and 0.4 were found, with higher saturation under the ribs. The saturated and the non-saturated states of the GDL samples were compared in order to quantify the dependence of gas phase permeability and effective relative diffusivity on liquid water saturation. Both these relative measures were found to follow power relationships of (1-s)(lambda), where the exponent. was approximately 3 for all cases except for the in-plane diffusivity where it was closer to 2. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.005209jes] All rights reserved.