The oxidation reactions of platinum(II) complexes of the type [Pt(en)L2]2+[en = H2NCH2CH2NH2, L = NH3, NH2R (R = Me, Et, Prn, CH2But, CH2Ph, Bun, C4H7, C5H9, or C6H11), 4-cyanopyridine, ½ MeHNCH2CH2NH2, ½ Me2NCH2CH2NH2, or ½ Me2NCH2CH2NMe2] and [PtL4]2+[L = NH3 or NH2R (R = Me, Et, or Prn)] by IrCl62– in the presence of chloride ions have been kinetically investigated in aqueous perchlorate media (pH 2.0–3.0, I= 1.0 mol dm–3). In the absence of IrCl63– the reactions obey the rate law –d[IrCl62–]/dt= 2 (ka+kb[Cl–])[complex][IrCl62–]. The iridium(III) reaction product, IrCl63–, lowers the rates. The results obtained are consistent with a redox mechanism involving a five-co-ordinate platinum(II) intermediate and two successive one-electron-transfer steps. The reactivity of the complexes is strongly dependent on the ligands co-ordinated to platinum(II), steric hindrance being the most important factor in the observed trend.