The reactions of mer-trichloro(diethylenetriamine)platinum(IV) chloride, mer-[Pt(dien)Cl 3)Cl, with a nucleobase, 9-methylhypoxanthine, mHyp, and a nucleotide, 5′-guanosine monophosphate, 5′-GMP, were studied to model those of platinum(IV) antitumor agents with nucleic acids. In dilute aqueous solution, 0.07 mM, mer-[Pt(dien)Cl 3]Cl reactions with 9-methylhypoxanthine or 5′-GMP require more than eight days at 37°C to show substantial product formation, while reactions with the analogous platinum(II) compound [Pt(dien)Cl]Cl are complete within 24 hours at room temperature. Reactions with mHyp or 5′-GMP at higher concentrations of mer-[Pt(dien)Cl 3]Cl and ligand are at least 50 times slower than the same reactions with [Pt(dien)Cl]Cl. Most interestingly, even though only platinum(IV) complexes and nucleobase(nucleotide) are used as reactants, all product mixtures show the presence of platinum(II) as well as platinum(IV) adducts. Proton NMR resonances corresponding to 9-methylxanthine, a possible oxidation product of 9-methylhypoxanthine, have been detected in reaction mixtures of mer-[Pt(dien)Cl 3)Cl with mHyp and are reported for the first time.