In this study reversible and irreversible voltage loss in a polymer electrolyte membrane fuel cell undergoing an open circuit voltage (OCV) durability test was studied. OCV durability testing is thought to promote chemical degradation of the electrolyte membrane material via radical attack and degradation of the catalyst layer. The results for degradation under constant relative humidity showed that voltage degradation rates measured in the first 20-50h after polarization curve measurement consisted of a reversible, or transient, and irreversible component. A steady voltage decay rate became evident after 50 h of operation. Comparison to the voltage decay rates obtained from polarization curves showed that the steady voltage decay rate was representative of irreversible voltage loss due to irreversible changes in materials as shown by crossover and active surface area measurements. This study highlights the necessity of understanding the difference between reversible and irreversible voltage decay rates since the reversible decay rates were found to be much higher than irreversible decay rates. (C) 2008 Elsevier B.V. All rights reserved.