The mechanism of gamma-radiolysis of cis-dichlorodiammineplatinum (II) (cis-Platinum) with guanosine-5'-monophosphate (L) complexes in aqueous solutions was investigated. The decomposition limiting yield G (-cis-Pt (NH3)2L2) in deaerated and nitrous oxide saturated solutions, determined by measuring the absorbance at 254 nm, was found to be 0.8 and 1.2, respectively. It was found from HPLC that, upon irradiation of the complexes, guanosine-5'-monophosphate, guanosine and guanine were released. Other products, such as sugarphosphate and sugar, could not be detected with this UV-method.