Three commercially available gas diffusion layers were investigated by 3D X-ray computed tomography (CT). The carbon fibers and the 3D structure of the gas diffusion layers were clearly resolved by this lab-based technique. Based on 3D structures reconstructed from tomography data, the macroscopic, anisotropic effective thermal conductivities of the gas diffusion layers were calculated by solving the energy equation considering a pure thermal conduction problem. The average in-plane thermal conductivity for all samples, the carbon cloth and the two carbon papers, is by about a factor 4-12 larger than the average through-plane thermal conductivity. Furthermore, a clear dependence of thermal conductivity on the porosity and - especially for the carbon cloth - also on the local orientation of the fibers was observed. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.