Ozonation of cisplatin [cis-diamminedichloroplatinum (II)] in non-buffered and buffered aqueous solutions at pH 9 was carried out at 303 K in a semi batch system at laboratory scale. The degradation of cisplatin and the formation of reaction products were analyzed by reverse phase high performance liquid chromatography. In the first two minutes of reaction practically all the initial cisplatin was depleted and only one major peak was observed in chromatograms under both experimental conditions. The structure of this main product was proposed after analysis by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The mutagenicity of reaction mixture at non-buffered pH 9 was evaluated by performing Ames test. Non-mutagenic effect was observed. The second-order reaction rate constant of cisplatin with hydroxyl radical at pH 9 and 303 K was determined.