Physical properties affecting transport processes inside the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells mainly depend on the microstructure of its pore space. The presented characterization of the complex structure of the pore space is based on geometric three-dimensional (3D) graphs, which are marked to display transport-related properties such as pore diameters. This representation of the open volume allows for an investigation of local structural characteristics by considering local tortuosity characteristics, pore sizes, and connectivity characteristics, respectively. The notion of local shortest path length through the pore space of the GDL is introduced and the probability distribution of this random variable is computed. Its mean value is related to the (physical) tortuosity, which is given by the ratio of the mean effective path length through the GDL and its thickness. The developed methods are applied to simulated and to real (experimentally measured) 3D data. The used stochastic 3D model for the GDL is an extended version of the multilayer model proposed by Thiedmann et al. [J. Electrochem. Soc., 155, B391 (2008)], including a more flexible modeling of binder. The numerical results show the sensitivity of the proposed local characteristics to varying binder modeling. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3222737] All rights reserved.