12CaO center dot 7Al(2)O(3) (C12A7) with a unique nano-porous structure and free O2- ions entrapped in sub-nanometer-sized cages is a fast oxygen-ion-conducting material. These free O2- may be replaced by various oxygen-related species, OH-, O2- and O-, by tuning the atmosphere during the heat treatment. We examined the conduction mechanism for stoichiometric C12A7 (C12A7: O2-). in which O2- ions exist as counter anions in sub-nanometer-sized cages, by Raman measurement of C12A7:O2- annealed in a dry O-18(2) atmosphere. It was revealed that the primary ion conducting species is an O2- ion which diffuses via exchange with O2- in the cage wall. An experimental result on the sample containing O- ions implied that O- is more mobile than O2- in C12A7. Ab initio calculations on the diffusion paths of O2- and O- ions in C112A7 supported the above experimental results. (C) 2008 Elsevier B.V. All rights reserved