A novel oxygen negative ion source named solid oxide ion source (SOIS) is proposed. To demonstrate its concept, thermionic emission of oxygen negative ions, O-, from a bare surface of yttria-stabilized zirconia (YSZ) was studied with a mass spectrometer capable of detecting negative ions. The emission current was investigated at the temperature range 750 to 950degreesC under pressures of about 2 x 10(-3) to 4 x 10(-4) Pa. O- ions proved to be emitted into a vacuum from the bare YSZ surface at elevated temperatures. The emission current increased with temperature and applied voltage. From Richardson-Dushman plots, activation energy of the thermionic emission was estimated to be about 2 eV. In continuous operation, the emission current decreased with time, finally approaching zero; however, after intervals, the current recovered. On the basis of the results, a model for explaining the emission mechanism of O- ions is proposed. The model is based on three key processes occurring on the surface of electrolytes: (1) formation of O- ions on the surface by electron capture of oxygen atoms provided by the migration of oxide ions across electrolytes, (2) accumulation of O- ions on the surface, and (3) emission of O- ions from the surface. (C) 2003 The Electrochemical Society.