The reaction mechanism for the primary reaction step of the hydroxylation of 3-fluoro-6-methylaniline, attacked at different positions (oxygen attack across a C-C bond and direct attack at positions para and ortho with respect to the NH2 group) catalysed by a high-valent ferryl-oxo porphyrin a(2u)-cation complex with H3CS- as an axial ligand, has been investigated on the basis of electronic structure calculations in local spin-density approximation. Non-repulsive potential curves are obtained only in cases of direct attack at the para- and ortho-positions with respect to NH2, but not for epoxic formation. Comparing the potential curves for the hydroxylation at the positions para and or ortho to the NH2-group, an attack at the para-position is more likely. The relative orientation of the substrate towards the porphyrin is essentially determined by the interaction between the substituents of the substrate and the porphyrin. Consequently, different geometrical orientations of the substrate are obtained for hydroxylation at the para- and ortho-positions. In both cases of direct attack the substrate plane is not parallel to the porphyrin plane. The decisive role of sulphur in the hydroxylation is demonstrated by the participation of the S(3p)-orbitals in all molecular orbitals involved in the reaction.