Durability of the proton exchange membranes (PEM) is a major technical barrier to the economic viability of stationary and transportation applications of PEM fuel cells. In order to reach Department of Energy objectives for automotive PEM fuel cells, a design lifetime of 5,000 hours over a wide temperature range is required. Reaching these lifetimes is an extremely challenging technical problem. Though good progress has been made in recent years, there are still issues that need to be addressed to assure successful, economically viable, long-term operation of PEM fuel cells. The lifetime is limited due to gradual degradation of both the electro-chemical and hygro-thermo-mechanical properties of the membranes. Eventually the system fails due to a critical reduction of the voltage or mechanical damage. However, the hygro-thermo-mechanical loading of the membranes and how this effects the lifetime of the fuel cell is not understood. The long-term objective of the research is to establish a fundamental understanding of the mechanical processes in degradation and how they influence the lifetime of PEMs. In order to reach this goal, we are testing polymer fuel cell membranes in our laboratory to determine their hygro-thermo-mechanical properties.