Performance decay in a polymer-electrolyte fuel cell can result from changes in the gas-diffusion layer, particularly in the micro-porous layer. Examples and characteristics of this decay will be presented here. Performance decay is greater with exposure to potential cycles than with exposure to constant high potential holds. Presumably, electrochemical oxidation of the carbon within the micro-porous layer is primarily responsible for this performance decay; it is known that transient potentials can accelerate the electrochemical oxidation of carbon. The implication that these results have on the development of advanced gas-diffusion layer materials is also considered. Finally, a recommendation for a standardized accelerated stress test protocol focused on the gas-diffusion layer is proposed.