Ion mobility and diffusivity coefficients were determined for cerium ions in Nafion XL perfluorosulfonic acid ionomer membranes at 100% and 50% relative humidity in a conductivity cell using a hydrogen pump. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). In order to decouple simultaneous effects of Ce ion mobility and back-diffusion which occur due to potential and concentration gradients, respectively, a one-dimensional model was developed and fit to these intermittent XRF profiles. The resulting mobility and diffusivity coefficients demonstrate the dramatic effects of potential and concentration gradients on Ce ion migration during PEM fuel cell operation. (C) The Author(s) 2017. Published by ECS. All rights reserved.