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Carbon-containing gases usually present when preparing carbon nanotubes, and can affect the field emission of carbon nanotubes. Water vapor is also an important kind of gas for field emission, concerned by both experimental and theoretical studies. Under strong electric field, the gas molecules may be decomposed to radicals. Using DMol3 code based on density-functional theory, we calculated the adsorption of the gas molecules CH4, CO and H2O and the CH3 and OH radicals under emission conditions. We found that the H2O and the methyl have advantages to field emission and the CH4, CO and the hydroxyl have disadvantages. The results of H2O and CO are consistent with experiments, and the enhancement of current by CH4 in experiment may be due to the methyl decomposed from the CH4. (c) 2006 Elsevier B.V. All rights reserved.