An in situ method of investigating the kinetics and mechanisms of oxygen surface exchange on (La0.8Sr0.2)(0.98)MnO3-delta (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) was developed by combining temperature programmed desorption, reaction, and isothermal switching with an isotopic tracer, O-18(2). The gas exchange behavior is separated into surface- and bulk-originated oxygen and compared to behavior predicted by chemical kinetics. A two-step mechanism is proposed consisting of dissociative adsorption of oxygen onto the surface followed by adsorbed oxygen incorporation into the material; different pO(2) dependences are obtained depending on which step is rate limiting. The rate-limiting step in LSCF is dissociative adsorption, whereas LSM is mostly incorporation limited at 600 degrees C and becomes more dissociative adsorption limited at 800 degrees C. The change in mechanism may be due to a phase transition and/or formation of oxygen vacancies. Overall, LSM is less catalytically active toward oxygen exchange than LSCF, with exchange onset occurring at 350 degrees C in LSM and 200 degrees C in LSCF. Above 700 degrees C, the activation energy for oxygen exchange in LSM and LSCF was found to be 0.90 and 0.10 eV, respectively. (C) 2008 The Electrochemical Society.