In this study the kinetics of two electron transactions between the reducing substrates glyoxylic and pyruvic acids and the oxidising complex cation, tris(biguanide)manganese(IV) in aqueous acidic media have been examined. Under the experimental conditions (pH = 1.50-3.78) only the fully protonated form of the oxidant reacts with the anions of the substrates whereas the parent acids, viz. glyoxylic and pyruvic acids, were found to be unreactive. The second-order rate constants for the oxidations of glyoxylate and pyruvate by the title MnIV species are (2.24 ± 0.10) × 10-1 and (5.40 ± 0.30) × 10-2 M-1S-1, respectively, at 25.0 °C and at I = 1.0 M (NaNO3 or NaClO 4). A rate-liming one-electron change between the redox partners seems justified. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.