This study investigates the oxidative transformation of diethylenetriaminepentaacetate (DTPA). a synthetic ligand. during drinking water ozonation. The rate coefficients for the reactions of CaDTPA(3-) and ZnDTPA(3-) with ozone were determined to be 6200 and 3500 +/- 150 M-1, respectively. The reactivity of Fe(III)DTPA(2-) towards ozone was found to be much lower (< 10 M(-1)s(-1)), but near neutral pH the reactivity of the Fe(III)-complexes is dominated by [Fe(III)(OH)]DTPA(3-). For the reaction of Fe[(III)(OH)]DTPA(3) with ozone a rate coefficient of 2.4 0.2 x 10(5)M(-1) s(-1) was measured. The rate coefficients of the reactions of the ZnDTPA-and Fe(III)DTPA with OH radicals have been determined by a competitive method as 2.4 +/- 0.4 x 10(9) and 1.5 +/- 0.1 x 10(9) M-1 s(-1), respectively at pH = 7. The degradation of low concentrations of DTPA complexes during ozonation was investigated in natural waters under drinking water relevant conditions. Based on our findings CaDTPA(3-) and ZnDTPA(3-) are judged as easily degradable. Fe(III)DTPA complexes showed a somewhat lower reactivity, but were still typically degraded by one order of magnitude at ozone dosages of similar to 20 muM (1 mg L-1) in the three natural waters tested. Molecular ozone was found to be the major oxidant for the metal-DTPA complexes during ozonation. (C) 2001 Elsevier Science Ltd. All rights reserved.