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An important challenge for PEMFC is stability and durability of the membrane separator. In this dissertation, we applied both experimental and modeling methods to investigate the chemical durability of PFSA membranes for fuel-cell applications. Degradation data were collected after Fenton's tests and the membrane samples were analyzed by XPS after Fenton's test; FTIR was also invoked to validate the XPS results. The effects of Fe2+ concentration and temperature on membrane degradation were discussed. The experimental results provide evidence of chemical attack of the CF2 backbone. Since the level of H2O2 was found to be key to membrane degradation, we designed a novel spectrophotometric method to quantitatively determine H2O2 concentration in a fuel cell by using a multilayer MEA. In addition, a model for H2O2 formation, transport, and reaction in PEMFCs is established for the first time to validate experimental data and study formation mechanism. The humidity effect on membrane degradation was studied by collecting vent water during the tests. The membrane conductivities and mechanical properties were measured by ex-situ high-throughput instruments. FTIR was applied to study both the formation of new groups and the relative abundance of existing groups in the degraded membrane. The thermal stability of degraded membranes was determined by TGA. The cross section of a degraded MEA sample was imaged with SEM to investigate the mechanical structure change. The effect of temperature on membrane degradation was also investigated. XPS spectra were collected from both anode and cathode sides of fuel-cell membrane to compare the effect of temperature on each side. Atomic analysis was performed to study the impact of temperature on both backbone decomposition and side group degradation. A multilayer MEA was used to study the effects of location and thickness on membrane degradation. An improved kinetic model of membrane degradation was built to simulate the experimental data. Finally, an attempt to mitigate membrane degradation by using peroxide decomposition reagent was performed. OCV curves were recorded during two fuel-cell durability tests with and without the addition of this additive. Both FER and TER were compared. Recommendations for the improvement of peroxide decomposition additive were suggested.