Reactions of [Pt(rac‐1,2‐DACH)Cl2], [Pt(1R,2R‐DACH)Cl2] and the anticancer drug oxaliplatin with the tripeptide glutathione and glutathione disulfide have been studied by HPLC, ESI‐MS and NMR methods. The same two major products are formed in each reaction: a thiolate‐bridged dimer and a novel thiolate‐bridged dinuclear macrochelate containing a nine‐membered chelate ring. Redox reactions of PtII anticancer drugs with biological disulfides are of potential importance to their mechanism of action. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)