The spatial distribution of cerium valence in H-2-reduced model planar Pd/Ce0.7Zr0.3O2 catalysts was determined by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Spherical reduction zones were found beneath each Pd particle, consistent with a Pd-catalyzed, oxygen anion diffusion-limited bulk oxide reduction process. Evidence was also found for a more extended surface reduction process, possibly associated with hydrogen spillover from the Pd particles. (C) 2013 Elsevier Inc. All rights reserved.