The degradation kinetics of the widely used iodinated X-ray contrast medium, namely diatrizoate, was investigated during ultraviolet (UV) photolysis and UV/chlorination. Factors influencing diatrizoate degradation, such as pH, chlorine dosage, and bromide concentration, were also evaluated. Diatrizoate was not degraded during chlorination. However, the degradation of diatrizoate during UV photolysis and UV/chlorination complied with the pseudo-first-order kinetics. At pH 7, the pseudo-first-order rate constant during UV/chlorination was 9.3 times higher than that during UV photolysis. The results of a radical quenching test using excessive tertbutanol suggested that the contribution of Cl-center dot, a type of reactive chlorine species (RCS), was limited. The contributions of UV direct photolysis, hydroxyl radicals ((OH)-O-center dot), and RCS to diatrizoate degradation during UV/chlorination at pH 7 were estimated to be 12.8%, 1.2%, and 85.9%, respectively. Results of further experiment suggested that (OCl)-O-center dot primarily caused diatrizoate degradation. As the chlorine concentration gradually increased from 25 to 150 mu M, the corresponding pseudo-first-order rate constant of UV/chlorination increased from 8.37 (+/- 0.42) x 10(-3) to 5.52 (+/- 0.28) x 10(-2) s(-1). When the pH value was increased from 5 to 9, the rate of diatrizoate degradation reached the maximum at pH 7 and gradually decreased during UV/chlorination; however, the rate of diatrizoate degradation remained stable during UV photolysis. The degradation of diatrizoate was inhibited when the bromide concentration was increased due to the competition for chlorine between bromide and diatrizoate; thus, the generation of (OCl)-O-center dot was limited. Compared with UV irradiation, UV/chlorination could reduce the formation of iodinated trihalomethanes (I-THM) after chlorination. However, dichloroacetonitrile (C2HCl2N), which is carcinogenic, was generated during UV/chlorination.