State-of-the-art cathodes for solid acid fuel cells (SAFCs) based on the crystalline electrolyte CsH2PO4 (CDP) are comprised of a proton-conducting CDP network coated by a vapor-deposited nanostructured catalyst. Pd-rich (85 at%Pd) Pt-Pd oxygen reduction catalysts vapor-deposited on CDP display both extraordinary activity for oxygen reduction and poor stability in cathodes for SAFCs operating at 250 degrees C. Similar catalysts with lower Pd content (57 at%Pd) are less active and more stable. Using X-ray absorption spectroscopy (XAS), we find that these catalysts are structurally similar and that structural variations are insufficient to explain the observed differences in activity. XAS and solid-state and solution nuclear magnetic resonance (NMR) also show that additional water-soluble chemical species are present in the Pd-rich electrode after fuel cell operation. We attribute the presence of these species to the reactivity of the Pd-rich catalyst with CsH2PO4 and suggest that these products are the cause of the observed deactivation. (C) The Author(s) 2016. Published by ECS. All rights reserved.