We report peculiar emission behavior of large-area cathode based on a multiwalled carbon nanotube/polymer nanocomposite. Mass spectra and kinetics of the partial pressure of the main volatile products released from the surface of the electrodes were obtained when a constant voltage of different levels was applied to the cathode. The main volatile products during field emission diode operation are H2, H2O, CO/C2H4, and CO2. The behavior of H2O peak intensity is characterized by increased inertia relative to sudden voltage changes.