By spectrophotometric scanning of reacting solutions, we have studied the processes which can occur when Oz is admitted to aqueous solutions containing the Ion [CoII(CN)5]3- in the concentration range 0.0002-0.4M, at 0�. A scheme is given to explain the formation of and inter-relations between [(CN)5CoIII-O-O-CoIII(CN)5]6-; [(CN)5CoIII-O-O-CoIV(CN)5]5-; [(CN)5CoIIIOH2]2- ; [CoIII(CN)6]3-, and a complex believed to be [(CN)5CoIII-O-OH]3-, which absorbs at 272 mp Preparative methods are described for obtaining several of the oxidation products in the crystaline state. Hydrolysis of [(CN)5CoIII-O-O-CoIII(CN)5]6- to [(CN)5CoIIIOH2]2- takes place via the 272 mμ complex, which is fairly stable in alkaline solution and is resistant to oxidation. The same 272 mμ complex can be formed more directly by oxygenation of the hydrido complex [(CN)5CoIIIH]3-, which is present in aged or hydrogenated solutions of pentacyanocobaltate(II). All of the pentacyano oxidation products can be reduced by sodium borohydride to [(CN)5CoIIIH]3-, which is further reduced by excess borohydride to an insoluble green compound and finally to metallic cobalt. We have briefly examined the reactions of ammonia and CN- with [(CN)5CoIIIOH2]2-, and the reaction of [(CN)5CoIII-O-O-CoIV(CN)5]5- with reducing agents, including [CoII(CN)5]3-.