The ionic activity at the interfaces of the membrane and the ionic polymer phase in the catalyst layers can have a significant impact on the transport rates of protons and water in a proton exchange membrane fuel cell (PEMFC). This study investigated the effects of some treatment processes on the surface ionic activity of extruded Nafion membranes and their performance in a PEMFC. The treatment processes included H2SO4 and H2O2 wash and plasma sputter and reactive ion etching. The membrane surface ionic activity was determined by the S:C ratio using X-ray photoelectron spectroscopy. The results showed that the surface of these membranes, as received and after H2SO4 wash, had lower ionic activity than that of its bulk (S:C=0.026 vs 0.053). Treatment with H2O2 had a significant impact on the surface ionic activity, lowering the surface S:C ratio further to 0.017. The Teflon-rich skin of extruded Nafion membranes could be removed by sputter etching with argon exposing a surface with higher ionic activity (S:C=0.047). However, reactive ion etching with SF6 and argon led to a further decrease in the surface ionic activity (S:C=0.011). Fuel cell results showed a strong correlation between the membrane surface ionic activity and the fuel cell performance.