The photocatalytic degradation in aqueous solution of aminopyrimidines (2-aminopyrimidine, 4-aminopyrimidine, 4-methyl-2-aminopyrimidine, 2,6-dimethoxy-4-aminopyrimidine) on TiO2 has been investigated. Their transformation pathways seem to be closely related to the position of the amino group in the heterocyclic ring; especially the fate of the organic nitrogen is deeply influenced. Both the ratio [NH4+]/[NO3-] and the extent of their formation are connected to the nature of the substituents and the amino group position in the heteroaromatic ring. When the amino group is located in C-4, the transformation pathways proceed mainly through the cleavage of the C-N bonds, with the formation of oxygenated structures as intermediates and the release of nitrogen mainly as ammonium ions. In this case, the stoichiometric conversion of nitrogen into inorganic ions and the complete mineralization is achieved within few hours of irradiation. On the contrary, when the amino group is held in C-2, a lack in both nitrogen and carbon mineralization is observed. It is attributable to the stoichiometric conversion of the N-C(NH2)-N moiety into guanidine, a very stable compound for which the mineralization is obtained only at long irradiation times (>70 h). Interestingly, in this latter case the nitrogen is mainly released as nitrate ions. (C) 2004 Elsevier B.V. All rights reserved.