Chlorine cation addition to benzene, aniline, anisole, styrene, chlorobenzene, and nitrobenzene was studied using NH3Cl+, ClC≡O+, protonated CH3Cl, and Cl+ as reagent ions. The reactions of protonated monochloramine were followed using a direct insertion membrane probe for sample introduction and a pentaquadrupole mass spectrometer for product characterization. The other reagent ions ClC≡O+, Cl+, and protonated CH3Cl were generated by electron ionization of acetyl chloride and carbon tetrachloride and by methane chemical ionization of CH3Cl, respectively. The main reactions of NH3Cl+ with aromatic compounds are electrophilic Cl+ and H+ addition and charge exchange to form the aromatic radical cation. Reactions of ClC≡O+ with aromatic compounds include (i) Cl+ addition, (ii) CO.+ substitution for a hydrogen atom, and (iii) formation of the molecular radical cation of the substrate. The naked Cl+ ion does not chlorinate aromatic compounds but does undergo charge exchange. Protonated CH3Cl also fails to add Cl+ to the aromatic compounds, proton transfer being the main reaction observed. Ion/molecule reaction products were characterized by comparing sequential product ion mass spectra (MS/MS/MS) to the MS/MS product ion mass spectra of reference ions, generated by chemical ionization of appropriate chlorine-substituted compounds. The sequential product spectra collected with the pentaquadrupole instrument show that both the Cl+ addition products and the CO.+ substitution products are σ-bonded to the aromatic compound. Comparisons with the MS/MS spectra of model ions suggest that both Cl+ and CO.+ add principally to the para position in aniline. Reaction occurs at the same position for anisole, although contributions from reactions at other sites are not excluded. Substitution of hydrogen by CO.+ in aniline and anisole also proceeds principally at the para position, although it also occurs at the nitrogen of aniline. Evidence is given for Cl+ binding to the beta-carbon in styrene and to the ring in chlorobenzene. Nitrobenzene, the least reactive compound, gave only traces of a Cl+ addition product and did not undergo substitution of CO.+ for hydrogen. However, it did display one unique reaction, the substitution of NO2. by Cl+. The evidence provided by the MS3 experiments for the site of Cl+ addition was tested against-and found to be consistent with-the sites predicted to have the highest Cl+ affinity by semiempirical AMI molecular orbital calculations.