The reaction of a variety of sulphur nucleophiles [thiourea, S-ethyl mercaptoethylamine (EMEA), glutathione (GSH), dimethylsulphoxide (DMSO) and methionine] with the platinum(II) complex [Pt(dien)Cl]+ has been studied at 25°C using the nucleophiles in large excess. The measurements confirm that the sulphur nucleophiles react directly with the platinum complex and that the solvolytic pathway makes little contribution to the reaction. The reactions were monitored by a combination of conductometric and spectrophotometric methods. The oxygen and nitrogen nucleophiles water and imidazole respectively have also been studied. Reagents such as thiourea, EMEA, methionine and glutathione are some 101–102 more reactive than the nitrogen nucleophile imidazole and some 102–103 more reactive than oxygen nucleophile. In a direct competition between sulphur (kS) and nitrogen (kN) nucleophiles for the platinum substrate, as will occur under biological conditions, the kS/kN ratio is at least 10 so that little of the nitrogen-substituted product will be formed in the reaction.