The scope and mechanism of the isomerization of arylamines to methyl-substituted aromatic heterocycles have been studied. Aniline, toluidines, naphthylamines and m-phenylenediamine all reacted to the corresponding ortho-methyl-substituted aza-aromatics when exposed to high NH3 pressure and elevated temperature in the presence of acid catalysts. Zeolites with a three-dimensional pore structure, especially H-ZSM-5, showed the best performance. Optimum reaction conditions are around 600 K and 10 MPa. Two mechanisms which had been proposed earlier for this apparent N-ortho C exchange reaction proved untenable. Neither incorporation of the N atom into the aromatic ring nor a mechanism based on an intramolecular Ritter reaction could explain the required high NH3 pressure or the product distribution. Two new mechanisms are proposed which can explain all observations. In both mechanisms, reaction starts with addition of NH3 to the arylamine, followed by ring opening. In one mechanism an alkyno-imine intermediate is formed; in the other mechanism an enamino-imine intermediate is formed through a reverse aldol reaction. In both cases ring closure and NH3 elimination lead to the required aromatic heterocycles. The high NH3 pressure is explained by the need to add NH3 to the aromatic ring, and the high temperature by the need to desorb NH3 from the acid sites.