The mechanism of oxygen exchange between the gas phase and Ba0.5Sr0.5Co0.8Fe0.2O3−δoxide was evaluated by considering the inhomogeneity of the oxide surface. The applicability of existing models for the analysis of the oxygen exchange mechanism was considered. A new model with a dissociation step was suggested. The rate-determining steps of the oxygen exchange process were revealed under different experimental conditions. The change in the rate-determining step occurred at 600-650 °C. The probable cause was considered taking into account the parameter of nonequivalency of adsorption centers. A relationship between the oxygen isotope redistribution rates and the rates of the elementary steps in a “gas phase-solid oxide” system was revealed.