Carboplatin aqueous solutions, with chloride ions added at different concentrations, were studied by X-ray absorption spectroscopy (XAS). The comparison of solid and solution spectra shows that carboplatin and cisplatin spectra are strongly different, and that the carboplatin ligands induce a specific structure of the spectrum, conserved in solution. Hence, it is possible to study by XAS the evolution of carboplatin in solution. This study shows that carboplatin is the major compound present in solution, even after 15 days, in neutral solutions with chloride concentration less than 9%, exposed to light or not. On the contrary, with high chloride concentrations (18%) or in acidic solutions (0.1 M HCl), the carboplatin is chlorolysed, the evolution of the solution composition can be followed by XAS and cisplatin formation is evidenced.