The thermodynamics and kinetics of oxygen radical formation in cages of a nanoporous crystal, 12CaO(.)7Al(2)O(3), were examined during isothermal annealing in a dry oxygen atmosphere. The formation of oxygen radicals, O-2(-) and O-, involves the outward diffusion of extraframework oxide ions, their 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 extraframework oxide ions provides the driving force for this process. Evaluating the radical formation enthalpy and entropy enabled us to determine the temperature and oxygen partial pressure dependences of equilibrium oxygen radical concentration. The rate-limiting process for the radical formation is not the surface reaction, but the total ionic diffusion process, in which the smaller diffusivity of O-2(-) likely dominates the process.