The development of high performance solid oxide fuel cells is dependent upon the fundamental understanding of the oxygen reduction process at the cathode surface. Isothermal isotopic switching is a promising technique used to reveal the mechanism of oxygen exchange on (La0.8Sr0.2)(0.98)MnO3 +/-delta (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) powders. Various temperatures, pO(2) and sample sizes were explored. The rate of oxygen exchange for LSM was determined to be insensitive to changes in pO(2) and strongly dependent on temperature. The opposite is observed in LSCF, which was insensitive to changes in temperature between 600-800 degrees C and strongly dependent on the pO(2). This behavior indicates that LSM is limited by incorporation of adsorbed oxygen atoms into the lattice and that LSCF is either gas phase diffusion limited or dissociative adsorption limited under operating conditions. A 2-step mechanism was used to model the isotopic exchange. Rate constants and simulated profiles were obtained using an iterative program to fit parameters from experimental measurements. The fit for LSCF was good for both surface and bulk behavior, however, bulk conversion for LSM did not agree with the predicted behavior. LSCF fit the behavior expected for a surface coverage limited reaction, where the surface reaction occurs more rapidly than the mass transport of the reactants to the surface. The conversion for LSM was slower than predicted by the model, suggesting that the diffusion of oxygen from the particle core to the surface is the actual rate limiting step. Degradation in LSCF samples was observed to occur after 20+ switching cycles; the reactivity difference was due to the reduction in the turnover frequency and not to a change in mechanism. (C) 2009 Elsevier B.V. All rights reserved.