A comparative kinetic study of the reactions of two mixed valence man- 4,7,10-tetraazacyclododecane] with thiosulfate has been carried out by spectrophotometry in aqueous buffer at 30 • C. Reaction between complex 1 and thiosulfate follows a first-order rate saturation kinetics. The pH dependency and kinetic evidences suggest the participation of two complex species of Mn III (µ-O) 2 Mn IV under the experimental conditions. Detailed kinetic study