The rate of N–H hydrogen-isotope exchange in NN-dimethylanilinium ions has been studied in the solvents sulphuric acid and deuteriosulphuric acid using the difference in the ¹H n.m.r. absorption of the NMe₂H and NMe₂D groups. With the N-deuterio-NN-dimethylanilinium ion, the reaction rate decreases steadily with acidity as expected from previous studies of the reaction mechanism at lower acidities. However, the rate of N–D exchange in the N-deuterio-NN-dimethyl-3,5-xylidinium ion reaches a minimum in 83% sulphuric acid and then increases steadily with the acidity. Under the same conditions, hydrogen-isotope exchange occurs in the aromatic ring. The effect of methyl substituents on the rate of the acid-catalysed N–H exchange indicates that this reaction and the C–H exchange have a common initial step involving the donation of a proton or deuteron to the positions in the aromatic ring ortho or para to the nitrogen. The Wheland intermediate, so formed, is considered to be able to lose a proton from either the carbon or the nitrogen. This interpretation of the acid-catalysed exchange is supported by the effect of isotopic substitution in the aromatic ring on the rate of N–H exchange and by preliminary studies on the range of this reaction mechanism.