The near-diffusion-controlled reactions of hydroxyl radical, hydrated electron, and hydrogen atom with platinum macrocyclic complex ions in aqueous media have been studied using pulse radiolysis in conjunction with UV-visible absorption and conductivity detection. The hydrated electron and hydrogen atom react with trans-[Pt(cyclam)(Cl)(2)](2+) where cyclam is 1,4,8,11-tetraazacyclotetradecane to yield platinum(III) transients that exhibit intense absorption peaks in the 280-300 nm region; however in the case of the H-atom, the reaction involves a competition between chloride abstraction and a minor process, suggested to he attack on the organic ligand. The platinum(III) products are kinetically labile toward loss of chloro ligands, but these reactions are reversible in the presence of added KCI. The reactions of hydroxyl radical with [Pt(cyclam)](2+) and with [Pt(tmc)](2+) where tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, lead to platinum(III) intermediates absorbing III the 250-300 nn region. Depending on the presence or absence of added KCl and on the pH, the platinum(III) cyclam systems can react to form a product(s) exhibiting absorption peaks near 330 and 455 nm, and this species is proposed to be a long-lived amidoplatinum(III) complex. In support of this proposal is the observation that the tmc system does not give rise to a similar visible-absorbing product. The interrelations of the cyclam-based transients through acid-base, chloro-substitution and water-elimination processes are discussed.