Emission behaviors of Spindt type molybdenum microtip field emitter arrays (Mo-tip FEA) are observed by use of a field-emission electron microscope (FEEM). Individual active micro-tips are separated as bright spots on a phosphor screen. Changes of both total-emission current and emission image are examined by introducing gases, which are nitrogen, hydrogen, methane, and ethylene, respectively. The number rates of active microtips behave differently from the total-emission current. The total-emission current increases during gas introduction, and then gradually decreases after the gas is evacuated. The final current levels are different from gas to gas. Among the gases, ethylene gives the largest final current, and the largest number of active microtips. On the contrary, methane decreases in both the final current and the number of active microtips. The emission behaviors for hydrocarbon are different from the nonorganic gases. (C) 2004 American Vacuum Society.