We have studied the effect of residual gas on the Mo and diamond-covered Mo field emitters. When the pressure increased by outgassing of the chamber, the emission current increased. The residual gas and thermal desorption analysis indicated that the hydrogen adsorption on the emitter was the reason for the increased current. The emission from each tip was examined using an aperture lens to magnify the electron beam. The gas-emitter interaction changed the number of tips that were emitting as well as the emission from the individual tip. When the tips were coated with nano-diamond particles, the pressure dependence was reduced significantly. (C) 2003 Elsevier Science B.V. All rights reserved.