In order to give insight into the guiding principle of further improvement in surface reactivity of mixed conducting cathode materials of solid oxide fuel cells, electrochemical impedance spectroscopy study was carried out on various LaCoO3- and (La,Sr)CoO3-based dense thin film electrodes with emphasis on the effect of oxygen partial pressure, Po-2. The area specific conductivity corresponding to the surface reactivity, which is evaluated from the major impedance component appearing at low frequency range, shows a Po-2-exponent of 1/2 irrespective of electrode material. After examining a series of possible elementary reaction steps in the oxygen reduction reaction, the universally observed Po-2 dependence can be explained only by assuming that the rate-determining step is the reaction where surface bidentate peroxide ion reacts with an oxide ion vacancy in the cathode to yield two O- ions, one left at the surface and the other in the cathode interior. The origin of the minor impedance component often observed at higher frequency range is also discussed on the basis of its independence on Po-2, which leads the authors to suggest that this impedance can be associated with the redox reaction between the surface O- and O2- in the cathode. (C) 2015 Elsevier B.V. All rights reserved.