A model for H2O2 formation, transport, and reaction in PEMFCs is established for the first time. Profiles of oxygen and H2O2 concentration inside the fuel cell are simulated using the agglomerate model for the electrode. The predicted concentration of H2O2 shows the same trend as experimental data under different conditions, but the level was only of the same magnitude. Low humidity, high temperature, and high oxygen/hydrogen partial pressures were found to increase the concentration of H2O2. An increase in membrane thickness or metal ion contaminant level reduces the concentration of H2O2 in the membrane. Lowering the oxygen permeability in the ionomer is the most important and effective method to reduce the formation of H2O2. The simulation results also show little change in H2O2 concentration while operating the fuel cell above 0.6V. Anodes designed with considerable thickness, high catalyst loadings and active areas can also help to suppress H2O2 formation. Finally, recommendations are made to mitigate the effects of H2O2 and prolong membrane lifetimes.