Empirical equations were developed to describe the voltage-time behavior of polymer electrolyte membrane fuel cell (PEMFC) stacks at constant current discharge. When either ambient temperature or discharge current is too high, the experimental voltage-time curves exhibit rapidly falling cell voltage within a short discharge time. Various experimental voltage-time curves have been fitted very well with empirical equations at different discharge currents and ambient temperatures. The effect of parameters of the empirical equations on the shape of voltage-time curve is also analyzed. Inadequate mass-transfer is likely a reason for the voltage falling down rapidly, and polymer electrolyte membrane dehydration is responsible for the inclination of the voltage-time curves. The empirical equations are helpful for forecasting and explaining the long-term discharge performance of the PEMFC stacks. Published by Elsevier Science B.V.