Investigation on oxidative mineralization of various surfactants over TiO2 photocatalyst reveals an anomalously retarded degradation of the trimethylammonium moiety in cationic surfactants. Model reactions using tetraalkylammonium chlorides as substrates confirm that the stepwise behavior of the mineralization time courses is ascribed to intramolecularly selective decomposition caused by a considerably slow degradation rate of the methyl groups directly bound to the quaternary nitrogen. A strong inhibition effect of bromide or iodide anions preventing the complete mineralization is also revealed. (C) 2009 Elsevier Science B.V. All rights reserved.