Manganese-catalysed ozonation of glyoxalic acid in aqueous solutions has been investigated in the pH range 2.0-4.0. The addition of manganese ions (Mn(II) or Mn(IV)) to a glyoxalic acid-ozone system allows the substrate to be oxidized through a mechanism different from that reported for uncatalysed ozonation, as underlined by the formation of methanoic acid. The presence of solid MnO2 results in a further increase in the system reactivity, High levels of mineralization are generally achieved during glyoxalic acid-catalysed ozonation,