The understanding of the oxygen incorporation mechanism at the cathode is crucial for the improvement of solid oxide fuel cell performance. To elucidate the reaction mechanism on the prototype cathode material Equation or symbol description not availableBa1xSrxCoyFe1yO3δ (BSCF), the reaction kinetics is studied on geometrically well-defined dense thin film microelectrodes by impedance spectroscopy. This avoids interference from the electrode morphology and allows a quantitative comparison of the oxygen incorporation rate for BSCF with different cation compositions. The correlation between the effective surface exchange rate constant Equation or symbol description not availablek extracted from the impedance spectra and various bulk properties is studied. With oxygen diffusion coefficients reliably measured by oxygen isotope exchange and secondary ion mass spectrometry (SIMS), a correlation between Equation or symbol description not availablek and the concentration and mobility of oxygen vacancies is confirmed. Together with the temperature and oxygen partial pressure dependence of Equation or symbol description not availablek, an oxygen incorporation mechanism on BSCF is proposed with a rate-determining step involving the migration of oxygen vacancies.