We report on the effects of adsorbates on the field emission properties of multiwall carbon nanotubes through current-voltage (I-V) measurements. Adsorbates were found to increase the field emission current of carbon nanotubes by as much as two orders of magnitude. At high emission current, Joule heating of the field emitting carbon nanotubes causes the emission of incandescent light and desorption of adsorbates due to heating of the field emitting carbon nanotube. We also found that electric field activated emission current jumps, probably due to the formation of enhanced tunneling configurations between adsorbate molecules and the carbon nanotube. (C) 2004 Elsevier B.V. All rights reserved.