This paper seeks to develop a structure-performance relationship for gas diffusion layers (GDLs) of polymer electrolyte fuel cells (PEFCs), and hence to explain the performance differences between carbon paper (CP) and carbon cloth (CC). Three-dimensional simulations, based on a two-phase model with GDL structural information taken into account, are carried out to explore the fundamentals behind experimentally observed performance differences of the two carbon substrates, i.e. CC and CP, under low- and high-humidity operations. Validation against polarization data is made under both operating conditions, and the results indicate that the CC is the better choice as a GDL material at high-humidity operations due to its low tortuosity of the pore structure and its rough textural surface facilitating droplet detachment. However, under dry conditions, the CP shows better performance due to its more tortuous structure, which prevents the loss of product water to dry gas streams, thus increasing the membrane hydration level and reducing the ohmic loss. The present work is one step toward developing a science-based framework for selection of materials for next-generation, high-performance gas diffusion media. (c) 2006 Elsevier Ltd. All rights reserved.