A corona-discharge reactor is applied to removal of CH3I from N-2 in the presence of H2O and O-2. In the reactor. CH3I is converted to negative ions, negative-ion clusters or particles by electron attachment or O-3 in the presence of H2O and I-2. In the removal of CH3I from N-2, since one electron contributes to remove about three CH3I molecules, radicals reactive with CH3I are thought to be produced by the dissociative electron attachment. The removal efficiency of CH3I is greatly improved by mixing H2O or O-2 with gas. In these removals, one electron removes about seven CH3I molecules from a N-2-H2O mixture, about ten CH3I molecules from a N-2-O-2 mixture, and about twelve CH3I molecules from a N-2-O-2-H2O mixture. A long term removal experiment indicates that the high removal efficiency of CH3I from the N-2-O-2 mixture is maintained for an extended period, even after the anode surface is covered with the deposit of removed CH3I. In the removal of CH3I from a N-2-O-2 mixture, O-3 reaction improves the removal efficiency. Also, the formationof negative-ion clusters is important for this high removal efiiciency. Ab initia MO calculations suggest that the negative-ion clusters, O-[CH3I]m (m=1,2,4), OH-[CH3I]m (m=1,2,4) and H-[CH3I]m (m=1,2), are produced with a high probability. It is also suggested that O radicals do not influence the removal of CH3I.