The authors have studied the field emission (FE) behavior of multiwalled carbon nanotube (MWCNT) yarn at different temperatures. By analyzing how the FE characteristics change with temperature and the residual gas spectra of the vacuum chamber, it was concluded that the intrinsic FE of MWCNTs follows the Fowler-Nordheim theory fairly well, and the deviation from the theory at low temperature (below similar to 700 K) is caused by the influence of adsorption. Water is deduced to be the dominant species that influences the FE. It was found that the adsorbents can be desorbed at high voltage due to the Joule heating of the FE current. The adsorption was deduced to be physical from the value of the desorption temperature. The FE current recovery time at different pressures shows that monolayer adsorption is enough to cause obvious FE current changes. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3449188]