The objective of this study was to analyze the effectiveness of gamma irradiation to transform diatrizoate, DTZ, (contrast medium) in aqueous solution determining the role of oxidizing and reducting species on DTZ degradation. The mechanisms involved in DTZ degradation have also been studied. Removal of 91.9% of the diatrizoate was achieved at a dose of 1000 Gy at initial concentration of 25 mg/L. The degradation rate depends on the initial concentration. The dose constant is independent of the dose rate in the studied range of 3.83-1.06 Gy/min and slightly depends on solution pH in the interval pH 2.0-10.0. Results obtained show that low H2O2 concentrations increase the efficacy of the process, and high concentrations markedly decrease the dose constant. The reaction constant of diatrizoate with the hydrogen radical was determined, obtaining a k(H center dot) (DTZ) value of (2.2 +/- 0.5) x 10(9) M-1 s(-1). DTZ irradiation by-products have been determined by HPLC-MS and an oxidation/reduction mechanism has been proposed. Three possible degradation pathways based on the major byproducts found at different doses have been proposed. Moreover, the results obtained indicated a further possible compound degradation pathway, which is based on decarboxylation by hydrogen radical or HO center dot radical with a subsequent attack by hydroxyl radicals against the ketone group. (c) 2012 Elsevier B.V. All rights reserved.