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The reactions of homo- and heterodisubstituted dihalobenzene radical cations with NH3 were investigated by FT-ICR spectrometry. A halogen atom X (X = Cl, Br, I) is substituted in a gas-phase nucleophilic ipso substitution, yielding haloanilinium ions. The reaction efficiency, i.e., the percentage of reactive ion-molecule collisions, ranges from < 0.006% for 1-chloro-4-iodobenzene radical cations to 18% for 1-bromo-2-chlorobenzene radical cations. The reactivity of the halogenated benzene radical cations was found to be governed by two structural parameters. First, the radical cations with a low ionization energy, i.e., iodobenzene and its derivatives, react especially slowly regardless of the reaction exothermicity. Second, the reactivity of all dihalobenzene radical cations is strongly influenced by the substitution pattern. The reactivity of isomeric radical cations is always highest for the 1,2-isomer and lowest for the 1,4-isomer. These results show that the nucleophilic substitution of the halo- and dihalobenzene radical cations by NH3 proceeds by a multistep reaction mechanism with a double-well potential energy surface. The rate-determining step is the addition of NH3 to the aromatic radical cation in the collision complex. The structural parameters influencing this reaction step can be analyzed by the reactivity model of polar reactions of Shaik and Pross. From this the most important feature of the substitution reaction of mono- and dihalobenzene radical cations with NH3 is the different charge localization in reactants and products. This results in a strong influence of the difference of the ionization energies of the halogenated benzene and NH3 and of the dipole moment of the halogenated benzene, i.e., the precursor of the ionic reactant, on the activation energy of the addition step.