Abstract The reaction products obtained from mixtures of 5'-GMP and platinum(IV) compounds with formula Pt(IV)Cl 4(LL) and Pt(IV)Cl 2(OH) 2(LL) (LL representing two monodentate or one bidentate amine ligand) have been characterized by proton NMR spectroscopy. The amines used are NH 3, H 2N-CH 2CH 2-NH 2 (ethylenediamine, en), H 2N-CH 2-C(CH 3) 2-CH 2NH 2 (2,2-dimethyl-1,3-diaminopropane, dmdap), and HC(CH 3) 2-NH 2 (isopropylamine, ipa). Conditions varied during the reaction are pH (values of 4, 7, and 10), effect of visible light, and addition of vitamin C as a reducing agent. In all cases, the major product appeared to be the bis(5'-GMP) (LL)Pt(II) compound. The pH effect is limited; i.e., at pH 4 the reactions proceed somewhat faster than at neutral pH, while at pH 10 slower reactions occur. The illumination with visible light also induces only slight differences in the yields of the products. On the other hand, when vitamin C is present, the reactions proceed quite rapidly, resulting in the same main product but in higher yields (up to 80%). The facts that apparently no Pt(IV) adducts with 5'-GMP can be observed under these conditions and that the major products are bis(5'-GMP) (LL)Pt(II) compounds clearly support the hypothesis that the antitumor activity of certain platinum(IV) compounds is based upon in vivo reduction to the corresponding platinum(II) compounds.