It is known that impurities, especially metal ions in feed water, can cause significant performance degradation of proton exchange membrane water electrolyser (PEM WE). In this study, the long-term effect of iron ion contamination on single cell performance is investigated by introducing Fe2 (SO4)3 into deionized water fed in PEM WE. Electrochemical impedance spectroscopy (EIS) and polarization curve results were recorded during the test. Results show that with 1 parts per million (ppm, molar ratio) Fe3+ contamination at the test condition of 0.5 A/cm2 and 60 °C, the cell performance degrades severely, especially the charge and mass transfer resistances increase significantly with time. Resistance values obtained through fitting the experiment data with equivalent circuit model were used to better describe the results. The results of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) test illustrate that the existence of Fe3+ promote the Fenton reaction, leading to the production of chemical radicals, which degrade the membrane and anode catalyst layer severely.