The gas diffusion layer (GDL) plays a very important role in the performance of Proton Exchange Membrane (PEM) fuel cells. The amount of compression on the GDL affects the contact resistance, the GDL porosity, and the fraction of the pores occupied by liquid water, which, in turn, affect the performance of a PEM fuel cell. In order to study the effects of GDL compression on fuel cell performance a unique fuel cell test fixture was designed and created such that, without disassembling the fuel cell, varying the compression of the GDL can be achieved both precisely and uniformly. Besides, the compression can be precisely measured and easily read out. Using this special fuel cell fixture, the effects of GDL compression on PEM fuel cell performance under various anode and cathode flow rates were studied. Two different GDL materials, carbon cloth double-sided ELAT and TORAY (TM) carbon fiber paper were used in these studies. The experimental results show that generally the fuel cell performance decreases with the increase in compression and over-compression probably exists in most fuel cells. In the low current density region, generally there exists an optimal compression ratio. (c) 2005 Elsevier B.V. All rights reserved.