Electron emission from silicon field emitters with a thin coating of diamond were studied during exposure to varying pressures of Ne, He, H-2, and D-2. Introduction of Ne and He at pressures > 10(-4) Ton : suppressed the emission current. Conditioning of held emitters in a 10(-5) Torr helium ambient improved the emissivity. After hydrogen and deuterium exposure, a continuous emission current was measured below the initial threshold voltages for electron emission. The effects of deuterium were significantly greater than for hydrogen. We believe this phenomenon is due to the formation of a surface dipole layer of hydrogen or deuterium, bonded to the surface carbon atoms, which lowers the electron affinity of the diamond surface.