The reactions of hydroxyl radical with tetraammineplatinum( 11) perchlorate and of the hydrated electron with transdihydroxotetraammineplatinum(1V) perchlorate have been investigated in aqueous media with the fast-reaction technique of pulse radiolysis in conjunction with conductivity and UV-visible absorption detection. The kinetic studies have been supplemented by final-product analysis on pulse-irradiated solutions for the formation of free ammonia and for changes in the concentration of platinum(I1). The hydroxyl radical reaction proceeds by an addition process to the platinum(I1) center, with a rate constant of (6.6 f 0.4) X lo9 M-l s-l. The hydrated e1ect;on reduces the platinum(1V) complex with a rate constant of (4.9 f 0.3) X 1O'O M-' s-'. The nascent products of these reactions are transitory species in which the metal has a formal oxidation state of 111, and evidence is presented to support their formulation as Pt(NH3)4(HzO)(OH)2' and Pt(NH3)4(0H)2+, respectively. Both species exhibit intense absorption bands with peaks near 270 nm, and they undergo reactions to yield three further transients, one of these being Pt(NH3)4(H20)23+ absorbing in the region of 250 nm. The two additional transients, arising at a secondary level, exhibit absorption bands of lesser intensity with peaks in the region of 340 and 530 nm. The nature of these species and their long-term reactions are discussed. A mechanism is proposed whereby the foregoing intermediates are interrelated by acid-base, ammonia substitution, and water elimination processes.