This paper reviews our recent activities of investigations on protons and other defects in ceria and their relations to the solid oxide fuel cell (SOFC) electrode reactions. Thermodynamic analyses have revealed that the chemical potential of dopant YO1.5 is a good measure for indicating the configuration of oxide ion vacancies in ceria and zirconia; that is, the large negative value of YO1.5 chemical potential in YSZ corresponds to the next nearest neighbor position of vacancies to the dopant site, whereas in doped ceria, the nearest neighbor position is favored and it corresponds to the less-negative chemical potential. Further thermodynamic analyses based on activities for CeO1.5 and YO1.5 have revealed that protons solubility and hole conductivity are governed mainly by the activity of YO1.5, whereas electron conductivity is well interpreted with CeO1.5 concentration calculated from the activities of CeO2 and CeO1.5. Analysis of the oxygen isotope exchange reaction rate that is enhanced in the presence of water vapor was made based on recognition that adsorption and desorption can be taken place at different sites which are connected with hopping processes; when a reaction-related elementary process is enhanced by water vapor, such a hopping process may become the rate-limiting step; this can explain why essentially the same activation energy was obtained for the surface reaction rate and the oxide ion diffusivity. In the electrochemical reactions, effects due to water vapors have been analyzed based on the fact that protons can be migrated in ceria due to the high proton solubility. For anodes, the oxygen transfer mechanism via water vapor from the electrolyte surface or from the electrolyte/anode/gas triple-phase boundaries to nickel surface is suggested under the assumption that protons in nickel can be transferred to ceria or consumed at the three-phase boundaries. This mechanism can explain why ceria can help to avoid the carbon deposition when hydrocarbon fuels are used. For cathode, enhanced oxygen surface reaction rate in the presence of water vapor can extend the electrochemical reaction sites from the triple phase boundary to the adjacent surface of the electrolyte. This may cause the change in oxygen potential distribution in the triple-phase boundary area; this can explain why ceria and zirconia show the different behavior against the chromium poisoning reported in literature. (C) 2004 Elsevier B.V. All rights reserved.