Perfluoro bis(sulfonyl)imides have potential for use in next generation ionomers for proton exchange membrane fuel cells. It is well known that these functional groups are strong acids and can be used to make membranes with excellent proton conductivity. Until now, there has been very little published work studying the long-term durability of these ionomers. In recent years, 3M has developed several imide-based ionomers, most notably a perfluoro imide acid (PFIA). Membrane electrode assemblies (MEAs) prepared using this ionomer have been run in the U.S. Department of Energy specified open circuit voltage (OCV) hold accelerated test. The purpose of this test is to stress the chemical stability of the membrane until a breach is detected. PFIA-based membranes, with mechanical supports and peroxide scavenging additives, will routinely exceed the 500-hour DOE target without crossover failure. However, a suppression in the OCV value has been observed in the first few hundred hours and, later, the cell resistance is observed to increase during this test. The first observation indicates a poisoning effect due to small molecule fragments on one or both electrodes. The resistance increase suggests a loss in protogenic groups and thus decreasing conductivity. Several techniques have been employed to better understand the origins of these observations. MEAs made with three membranes allows for the center layer to be subjected to the in-cell OCV test conditions and then separated at the end of life for analyses by FTIR, ¹⁹F NMR, and AC impedance measurements. Analysis of the effluent water by liquid chromatography - mass spectroscopy (LC-MS) is used to search for ionomer fragments. These methods have been employed to study the PFIA ionomer in both the membrane and electrode. In addition, an ionomer containing only the bis(sulfonyl)imide functional group has been studied. A complicated picture is emerging regarding the chemical stability of this class of imide-based ionomers and membranes.