We explored the N isotope fractionation associated with the oxidation of substituted primary aromatic amines, which are often the position of initial attack in transformation processes of environmental contaminants. Apparent 15N-kinetic isotope effects, AKIEN, were determined for the oxidation of various substituted anilines in suspensions of manganese oxide (MnO2) and compared to reference experiments in homogeneous solutions and at electrode surfaces, as well as to density functional theory calculations of intrinsic KIENfor electron and hydrogen atom transfer reactions. Owing to the partial aromatic imine formation after one-electron oxidation and corresponding increase in C-N bond strength, AKIEN-values were inverse, substituent-dependent, and confined to the range between 0.992 and 0.999 in agreement with theory. However, AKIEN-values became normal once the fraction of cationic species prevailed owing to 15N- equilibrium isotope effects, EIEN, of 1.02 associated with N atom deprotonation. The observable AKIEN-values are substantially modulated by the acid/base pre-equilibria of the substituted anilines and isotope fractionation may even vanish under conditions where normal EIE N and inverse AKIEN cancel each other out. The pH-dependent trends of the AKIEN-values provide a new line of evidence for the identification of contaminant degradation processes via oxidation of primary aromatic amino groups.