The presence of protons in mixed conducting cathode materials contacted with proton conducting electrolytes introduces additional possible mechanistic pathways for the surface oxygen reduction reaction compared to cathodes on oxide ion conducting electrolytes. A reaction network that includes these additional paths with protonated intermediate species is developed. The corresponding rate equations are derived, reflecting the point defects' contributions to the overall oxygen and water partial pressure dependencies. An equivalent circuit is derived for the general case of electrodes that exhibit proton, oxygen ion and electronic conductivity on proton conducting electrolytes. A characteristic circuit element is a chemical capacitance comprising proton and oxygen ion concentration changes. Complications due to nonnegligible electronic transference numbers arising in proton conducting electrolytes at high pO(2) are discussed. (C) The Author(s) 2015. Published by ECS. All rights reserved.