Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fluoride emission rate (FER) determined by fuel cell effluent water analysis was used to quantify the membrane degradation. Membrane degradation is most likely caused either directly or indirectly by the species formed as a result of the H-2 and O-2 reaction on the catalyst. To further understand the mechanism, the effects of the catalyst location, type, its interaction with O-2 and H2O, and cell current density on the FER were investigated and their implications on the underlying membrane degradation mechanism are discussed.