Emission of oxygen negative ions O- from a bare surface of yttria-stabilized zirconia (YSZ) was studied at 925 degrees C with a quadrupole mass spectrometer capable of detecting negative ions. The use of the mass spectrometer allowed one to identify O- ions, thereby enabling the investigation of emission stability of O- ions. Experimental results demonstrated that O- ions were emitted from the surface of YSZ continuously over 120 h. Although emission rate of O-ions decreased gradually with time, the emission rate of O- ions was confirmed to be temporarily restored after intervals. It was found that time constant of decay curve of O- ions was different from that of emission current. An analysis of the decay curves indicated that the difference in the time constants would be due to emission of electrons. Further, the analysis indicated that the number of emitted electrons would decrease with time more rapidly than O- ions, suggesting that it would be attributed to a variation in surface density of chemisorbed oxygen species on emission surface. (c) 2006 American Vacuum Society.