We have examined electric field-assisted thermionic emission of oxygen radical anion (O-) and electron from a stoichiometric 12CaO center dot 7Al(2)O(3) (C12A7). Lattice framework of C12A7 is composed of subnanometer-sized cages that can accommodate extra- framework species with negative charge, such as O-, O-2(-) ions, and electrons. Absence of oxygen gas supply leads to a steady coemission of O- ion and electron with a ratio of similar to 3:7. However, the oxygen gas supply markedly enhances the O- ion emission and suppresses the electron emission, realizing a sustainable and high-purity O- ion emission with a current density up to 11 nA cm(-2) at 825 degrees C. The O- ion emission accompanies a weak O-2(-) ion emission, which is smaller by similar to 1/30. The O-2 pressure at the rear side controls the concentrations of O- ion and electron on the emission surface as well as their concentrations in the bulk, and in turn, determines their emission intensities.