The performance of polymer electrolyte fuel cells (PEFCs) is substantially influenced by the morphology of the gas diffusion layer. Cells utilising sintered gas diffusion layers made with a low pore volume Acetylene Black carbon, at an optimised thickness, showed better performance compared with cells containing Vulcan XC-72R carbon. The cells were optimised using both oxygen and air as oxidants showing that different conditions were required in each case to achieve optimum cell performance. A model, in which the hydrophobicity and porosity of the diffusion layer affect water impregnation and gas diffusion through the gas diffusion layer, is presented to explain the influence of the diffusion layer morphology on cell performance.