Mn(II)(aq)-superoxide complex, MnO2+, was formed in pulse radiolysis by three distinct routes : Mn(I) + O-2, Mn(II) + O-2(-) and Mn(III) + H2O2. The stability of this complex was found to be governed by the two equilibria : Mn2+ + O-2(-) reversible arrow MnO2+ (1,-1) and Mn2+ + HO2 reversible arrow MnO2+ + H+ (6,-6). Both forward and reverse rate constants of the reactions involved in these equilibria were determined : k(1) (1.5 +/- 0.2) x 10(8) M(-1) s-(1), k(-1) = (6.5 +/- 1.0) x 10(3) s(-1); k(6) = (1.1 +/- 0.2) x 10(6) M(-1) s(-1), k(-6) = (6.5 +/- 1.0) x 10(6) M(-1) s(-1), yielding K-1,K--1 = (2.3 +/- 0.5) x 10(4) M(-1) and K-6,K--6 = 0.17 +/- 0.06. The metal-oxy complex MnO2+ decays by self-reaction with k(MnO2+ + MnO2+) = (6.0 +/- 1.0) x 10(6) M(-1) s(-1) and in acid solutions also by reaction with HO2, k(MnO2+ + HO2) = (1.0 +/- 0.3) x 10(7) M(-1) s(-1). In both cases stoichiometric amounts of H2O2 are formed as the end product. MnO was formed by reduction of Mn2+ with H atoms. It has an absorption spectra with maxima at 290 and 340 nm with epsilon(290) = 1300 +/- 200 M(-1) cm(-1) and epsilon(340) = 1000 +/- 150 M(-1) cm(-1). It reacts with oxygen with k(Mn(I) + O-2) (6.0 +/- 1.0) X 10(6) M(-1) s(-1). Mn(III) reacts with hydrogen peroxide with k(Mn(III) + H2O2) = (2.8 +/- 0.3) x 10(3) M(-1) s(-1).