The ion-molecule reactions of disubstituted benzenes with the ion system of acetyl chlorideunder the chemical ionization condition were examined and the fragmentation reactions of theadduct ions formed by the ion-molecule reactions were studied by using collision-induced dissocia tion technique. It was found that the electron-releasing groups favored the adduct reactions and the electron-withdrawing groups did not. The position and properties of substituting groups had an effect on the relative abundance of the adduct ions. The fragmentation reaction of the adduct ions formed by ortho-benzene diamine with the acetyl ion was similar to the reductive alkylation reaction of amine in condensed phase.