The successive formation of peroxomolybdenum(VI) complexes is discussed and it is shown that proton-consuming steps are followed by a proton-producing one; the pH maximum (=10) appears at a [H2O2]/[MoO4](2-) ratio of 2:1. The approximated formation constants are utilized to calculate the concentrations of peroxo complexes as a function of the ligand concentration. The mono-, di- and tetraperoxo complexes are formed in only negligible quantities, whereas the oxodiperoxohydroperoxo (simply, triperoxo) complex proves to be the predominant species. The triperoxo complex may react directly if a sufficiently reactive partner is present; if not, it oxidizes H2O2 to O-1(2), which reacts further. The activation parameters of formation of O-2 are determined as a function of pH: the enthalpy minimum (50.45 kJ mol(-1)) is found at pH 10, where the negative activation entropy reaches its lowest value (-149.5 J K-1 mol(-1)). The possible functions of the peroxo ligands are considered. (C) 2010 Elsevier B.V. All rights reserved.