The incorporation of cerium and manganese ions in perfluorosulfonic acid (PFSA) membranes strongly decreases the fluoride ion emission rate from fuel cell membrane electrode assemblies through their scavenging of reactive oxygen species generated during fuel cell operation. Concentration gradients of these ions and water fluxes lead to their migration and even loss from the fuel cell, but little is known about these phenomena that nevertheless impact proton exchange membrane fuel cell durability. We have determined the diffusion behavior of manganese and cerium ions in perfluorosulfonic acid membranes and find the diffusion coefficient of divalent manganese ions to be higher than that of trivalent cerium. We relate this observation to their relative efficacy in mitigating membrane degradation in a proton exchange membrane single cell, where ionic HPLC analysis of effluent water, technique used here for the first time, shows that three times greater loss of manganese ions than cerium ions after 200 hours at open circuit. Fluoride emission from PFSA membranes degraded ex situ and in situ are corroborated by results from detailed infrared, F-19 MAS NMR and X-ray photoelectron spectroscopic analysis. (C) The Author(s) 2018. Published by ECS.