The relative rate of CH bond activation by the Pt(II) ion decreased in the order HCH 2CH 3 > HCH 2CH 2 OH > HCH(OH)CH 3. The platinum(II)-ethylene complex, [PtCl 3(C 2H 4)] −1, 1, was the key intermediate in the oxidation of ethane, ethanol, and diethyl ether to 1,2-ethanediol by platinum(II) in aqueous medium. In particular, the intermediary of 1 in the oxidation of ethanol to 1,2-ethanediol and 2-chloroethanol was verified through labelling studies. In D 2O, 1, upon oxidation with one of a number of oxidants, converted to [PtCl 5(CH 2CH 2OD)] 2−, 2. 2 in turn decomposed to a mixture of 1,2-ethanediol and 2-chloroethanol on heating. The rate conversion of 1 was a function of pH, the anions present, and the oxidant used. While the conversion of 1 to 2 involved a nucleophilic attack by water for hydroxide ion), such a step was not observed in the absence of an oxidant. On basic D 2O, the sequential replacement of Cl − by OD − in 1 occurred to successively form [PtCl 2(OD)(C 2H 4)] −, [PtCl(OD) 2(C 2H 4)] − and [Pt(OD) 3(C 2H 4)] −. The process was reversed upon acidification. The species [PtCl 5(CH 2CHO)] 2−, 3, appeared to be the source for the small quantities of hydroxy- and/ or chloroacetaldehyde formed during the oxidation of 1. 3 was synthesized independently by the reaction of acetaldehyde with a mixture of PtCl 4 2−, and PtCl 6 2− in aqueous medium. When 1 was oxidized by Cl 2 in CD 3OD solution, the principal product was [PtCl 5(CH 2CO 2D)] 2− 4, when a small amount of water was present, and CD 3OCH 2CH 2OCD 3 in the absence of water.