The acid proton exchange rates of solutions of some halo-substituted anilines in trifluoroacetic acid have been determined by measuring the acid proton signal width. In addition, the proton exchange rate of protonated N,N-dimethylaniline in trifluoroacetic acid is discussed. It is suggested that proton exchange occurs via the following reaction:[Formula: see text]The reaction is believed to occur in two steps: the oppositely charged ions form ion pairs which then transfer protons internally. Experimental evidence indicates that the ion–pair dissociation constant is less than, or equal to, one.The activation energies range from 10 to 24 kcal/mole. The rate constants at 25 °C for a possible bi-ionic reaction are calculated. Ionic activity coefficients are estimated by the Debye–Hückel equation. Another set of rate constants is calculated, assuming that the ions exist partly or completely as ion pairs.The free energies, enthalpies, and entropies of activation are calculated for an ion-pair reaction and for an overall bi-ionic reaction. The free energies of activation range from 10 to 17 kcal/mole and for haloanilines are less than that of aniline. In the ortho- and para-substituted anilines, ΔF≠, in general, decreases with decreasing halogen electronegativity. ΔF≠ values for metasubstituted anilines decrease with increasing substituent electronegativity. ΔF≠ values are largest for parahaloanilines and smallest for orthohaloanilines. The ΔF≠ values for haloanilines can be rationalized by reasonable combinations of inductive and mesomeric effects. The values of entropies of activation are determined largely by the positions of the substituents. ΔS≠ values range from 35 to −10 entropy units (e.u.) with aniline and para-substituted anilines having the largest positive values and orthosubstituted anilines having negative values. The value of ΔF≠ for protonated N,N-dimethylaniline is larger than that for protonated aniline, as would be expected. ΔS≠ is quite small. This is attributed to steric hindrance of solvation of the dimethylanilinium ion.