The effect of O2 exposure on field emission properties of ultrananocrystalline diamond (UNCD) coated ungated Si tip arrays is reported. The behavior of current degradation during O2 exposure and current recovery following evacuation of O2 strongly depended on the initial current/electric field applied to the emitters. At a low initial current of 50 μA/electric field of 5.8 V/μm, O2 exposure (1×10-7, 5×10-7, and 1×10-6 Torr) resulted in a rapid decrease in current in the beginning, followed by a slow decrease. The current made a strong recovery toward the original value when O2 was pumped out. With an increased initial current of 180 μA/electric field of 10.3 V/μm, the currents recovered only to a lesser extent after O2 was evacuated. At an even higher initial current of 930 μA/electric field of 20.5 V/μm, the current decreased steadily and finally stabilized after an exposure of 720 L with only 34% degradation. To explain these observations, we propose a hypothesis of oxygen interaction with the UNCD surface. © 2003 American Vacuum Society.