The thermodynamics and kinetics of oxygen radical formation process in a nanoporous crystal. 12CaO . 7Al(2)O(3), were examined by isothermal annealing experiments in dry atmospheres. It is clarified that the formation of oxygen radicals. O-2(-) and O-, in the crystal occurs in the following three steps; outward diffusion of extra-framework oxide ions, its oxidation by absorbed oxygen molecules to form O- and O-2(-) at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extra-framework oxide ions is essentially the driving force for the process. Further, the analyses allow to predict the total oxygen radical content for a given annealing condition (C) 2004 Elsevier B.V. All rights reserved.