First-principles electronic structure calculations were undertaken to clarify the observed n-type behavior of the hydrazine (N2H4) adsorbed on zigzag carbon nanotubes (CNTs). It was found that, only in the presence of moisture, the adsorption of the hydrazine will introduce occupied impurity states near the bottom of the conduction band of the N2H4/CNT system, leading to the n-type behavior. To gain further insight into the role played by the moisture, an extension of this study to other gases adsorbed on CNTs was also performed. Our results suggest that humidity could affect experimental measurements of responses of CNTs to adsorbed gases. (C) 2011 Elsevier B.V. All rights reserved.