Substitution reactions of the complex [Pt(dien)I]⁺, where dien = diethylenetriamine or 1,5-diamino-3-azapentane, with the sulfur-containing rescue agent l-cystine have been studied in a 1.0 × 10^–1 mol dm^–3 aqueous perchlorate medium at various temperatures (25–50°C) and pH (2.30–1.00) using a UV-visible spectrophotometer. The products were characterized by their infrared and ¹H NMR data at various temperatures. These data indicate that [Pt(dien)I]⁺ formed a complex with l-cystine through Pt–S bonds at pH 1.00–2.30. This Pt–S bond is observed at 50°C with ring closure of the dien (δ 3.8–3.9, 2H, CH₂) and with open-ring dien (δ 3.2, 3.6–3.8 (dien), 2H, CH₂). All reactions follow the rate law –d[mixture]/dt = (k₁ + k₂[Nu])[Pt^II], where k₂ denotes a second-order rate constant and [Nu] is the total concentration of nucleophile. The product formation rate constant and activation parameters E_a, ΔH^#, and ΔS^# have been determined.