Many platinum (II) compounds containing cis-amine ligands exhibit anti-tumor activity. At the same time, these anti-tumor drugs have high renal toxicity in part due to the high affinity of platinum (II) to protein-sulfur sites. Intensive research has been done during last decade on reactions of cis-plantin with amino acids and proteins containing thiol groups. But little work has been done on oxidation reactions of such platinum (II) thiolato complexes, even though both the metal and sulfur sites are candidates for oxidation. We report here that platinum (II) cysteamine complex [Pt(II)(bpy)(S-cys)]⁺Cl^- reacts with singlet oxygen in D₂O leading to formation of sulfenato complex [Pt(II)(bpy)(SO-cys)]⁺Cl^- and sulfinato complex [Pt(II)(bpy) (SO₂-cys)]⁺Cl^-. The reaction has been monitored by ¹H NMR, ¹⁹⁵Pt NMR and FTIR. IR spectra show two sharp peaks at 1169 cm^-1 and 1209 cm^-1, typical S bounded Metal-S-O₂ stretches as reported before, arising from a sulfinate ligand bound to platinum through sulfur. The sulfenato complex is initially formed while the sulfinato complex is the ultimate product after 3 hrs indicating the oxidation process is sequential. One possible pathway of the reaction may be via formation of metal peroxo intermediates. However, the ¹⁹⁵Pt NMR spectra indicate oxidation occurs at the sulfur atom, not at the platinum. The kinetics of the reaction and trapping of potential intermediates will also be presented.