Pt(IV) anticancer active complexes are commonly regarded as prodrugs, and the reduction of the prodrugs to their Pt(II) analogs is the activation process. The reduction of a cisplatin prodrug cis-[Pt(NH 3 ) 2 Cl 4 ] and a carboplatin prodrug cis,trans-[Pt(cbdca)(NH 3 ) 2 Cl 2 ] by DL-homocysteine (Hcy) has been investigated kinetically in a wide pH range in this work. The reduction process follows overall second-order kinetics: −d[Pt(IV)]/dt = k [Hcy] tot [Pt(IV)], where [Hcy] tot stands for the total concentration of Hcy and k pertains to the observed second-order rate constants. The k versus pH profiles have been established for both prodrugs. Spectrohotometric titrations reveal a stoichiometry of [Pt(IV)]: [Hcy] tot = 1:2; homocystine is identified as the major oxidation product of Hcy by high-resolution mass spectrometry. A reaction mechanism has been proposed, which involves all the four protolysis species of Hcy attacking the Pt(IV) prodrugs in parallel. Moreover, these parallel attacks are the rate-determining steps, resulting in a Cl + transfer from the Pt(IV) prodrugs to the attacking sulfur atom. Rate constants An Accumet Basic AB15 Plus pH meter, equipped with an Accumet AccutupH R combination pH electrode (Fisher Scientific, Pittsburgh, PA), was used to measure the pH values of buffer solutions. Each time, the electrode was calibrated using the standard buffers

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