Carboxylation of hydroxide coordinated to Pt(IV) by anhydrides, pyrocarbonates, and isocyanates to form the corresponding Pt(IV) carboxylates, carbonates, and carbamates is described. For example, the acylation with acetic anhydride of (OC-6-33)-amminedichloro(cyclohexanamine)dihydroxyplatinum(IV) leads to formation of (OC-S-43)-bis(acetato-O)amminedichloro(cyclohexanamine)platinum(IV) (JM-216) in 60% yield. This compound is currently in worldwide clinical trials as an orally active antitumor agent. Pt(IV) dicarbonates and dicarbamates are prepared similarly by reaction of a Pt(IV) hydroxide with a pyrocarbonate or isocyanate. The carboxylation reaction can be used to prepare molecules containing ligands with pendant functional groups that would be difficult to introduce by substitution reactions. Thus (OC-6-43)-amminedichloro(cyclohexanamine)bis((methylthio)acetato-O)platinum(IV) was prepared, which was oxidized to the corresponding sulfoxide (OC-6-43)-amminedichloro(cyclohexanamine)bis((methylsulfinyl)acetato-O)platinum(IV). Finally, unsymmetrical carboxylate complexes may be obtained by reaction of a binary mixture of two electrophiles with a Pt(IV) hydroxide followed by chromatographic separation of the carboxylation products. A simplified synthesis of the K[(PtCl3NH3)-Cl-II] in 55% yield from cisplatin is also reported. This improves the availability of molecules of the general formula cis-Pt-II-Cl(2)AA’ (A, A’ = ammine, amine) which are critical intermediates in the multistep synthesis of the Pt(IV) carboxylates having antitumor activity.