The title compound, C(19)H(18)N(4)O(4)S, exists as a zwitterion in the solid state, with nominal proton transfer from a phenol group to the imine N atom. The 2,3-dihy-droxy-benzaldehyde fragment is oriented at a dihedral angle of 35.51 (11)° to the adajacent aniline group and makes a dihedral angle of 76.99 (6)° with the 4,6-dimethyl-pyrimidin-2-amine group. Intra-molecular O-H⋯O and N-H⋯O hydrogen bonds close S(5) and S(6) rings, respectively; the same O atom accepts both bonds. In the crystal, polymeric chains along [001] are formed from mol-ecules joined end-to-end by N-H⋯O and O-H⋯N hydrogen bonds; these feature R(2) (3)(6) loops. The polymeric chains are linked by C-H⋯O inter-actions and there are π-π inter-actions between the pyrimidine rings with a centroid-centroid distance of 3.446 (2) Å.