Investigations of the impact of Cr2O3 overlayers on the oxygen self diffusion in two SOFC materials were conducted to gain insight into the Cr poisoning mechanism at the cathode side of solid oxide fuel cells (SOFCs) with stainless steel interconnects. High density Y0.15Zr0.85O2 (YSZ) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) sintered pellets were covered with 3 to 30 nm Cr overlayers that were subsequently oxidized, forming Cr2O3. Standard O-18 tracer diffusion experiments at 800 degrees C were performed and ToF-SIMS profiling revealed that the oxygen ion diffusion coefficients were unaffected by the thin Cr2O3 overlayers, which is predictable since they are a bulk property, but the extracted effective surface exchange coefficients varied with Cr2O3 overlayer thickness. Solid-state reaction measurements and electronic structure considerations concerning the surface exchange, led to the conclusion that the observed oxygen uptake hindrance for Cr2O3 capped LSCF and the slight increase of the surface exchange coefficient for Cr2O3 capped YSZ can be attributed to the electronic properties of Cr2O3. A critical thickness for Cr2O3 was determined to be 12 nm where the transition from decreasing cathode-performance to a Cr2O3-property-governed regime occurs. (C) 2010 Elsevier B.V. All rights reserved.