Transformation of the pharmaceutical oxcarbazepine (OXC), a keto analogue of carbamazepine (CBZ) was investigated under different water disinfection processes (ozonation, chlorination and UV irradiation) to compare its persistence, toxicity and degradation pathways with those of CBZ. Analysis by LC-ion trap-MS(n) allowed for the identification of up to thirteen transformation products (TPs). The major abundant and persistent TPs (10,11-dihydro-10,11-trans-dihydroxy-carbamazepine (DiOH-CBZ), acridine (ACIN) and 1-(2-benzaldehyde)-(1H, 3H)-quinazoline-2,4-dione (BQD)) were identical to those previously reported during water treatment of CBZ. Only one new compound arising from an intramolecular cyclisation reaction was identified during UV irradiation. OXC reacted quickly with hydroxyl radical and relatively rapidly with free chlorine while slow reaction rates were recorded in presence of ozone and upon UV irradiation. An increase of the acute toxicity of UV irradiated solutions, monitored by a Daphnia magna bioassay, was recorded, probably due to the accumulation of ACIN. The formation of ACIN is of concern due to the carcinogenic properties of this chemical. ACIN was also generated during the direct UV photo transformation of DiOH-CBZ and 10-hydroxy-10,11-dihydro-carbamazepine (OH-CBZ), two metabolites of OXC and CBZ widely detected in water resources. Analysis of tap water samples revealed the occurrence at ng/L levels of the major TPs detected under laboratory scale experiments, except ACIN.