For solid oxide fuel cells with proton-conducting electrolytes, oxygen is reduced as well as water is formed at the so-called H-TPB where proton, oxygen and electron are available. Proton conductor cooperation to the cathode can thus increase the H-TPB length while oxygen-ion conductor could not. However, previous reports show that oxygen-ion conductor can also significantly increase the cathode performance, suggesting different cathodic mechanism, which is proposed in this work. Oxygen is reduced at O-TPB where oxygen-ion, oxygen and electron meet while H2O is formed at the electrode electrolyte interface. Experimental investigation reveals that the cathodic reactions are primarily limited by the diffusion of O-ad(-) at O-TPB and oxygen-ion transport within the electrode whereas water formation at the interface is not a limiting step. It is further exhibited that Sm0.5Sr0.5CoO3-delta electrocatalyst cooperated with SDC (Ce0.8Sm0.2O2-delta), an oxygen-ion conductor, show even higher cathodic performance than that with BCS (BaCe0.8Sm0.2O3-delta), a proton conductor, when BCS is used as the electrolyte. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.