The isotopic fractionation of oxygen in the oxidation of water by Co(III), Ni(IV), Cu(III) and Fe(phen)³⁺₃ has been measured as well as that involved in the oxidation of water by hypochlorite and persulphate catalyzed by divalent cobalt, nickel and copper ions. In all reactions studied, the evolved oxygen is depleted in O¹⁸, fractionation factor ranging from 1.0022 to 1.0455. In the metal catalyzed reactions the same fractionation factor (O¹⁶/O¹⁸= 1.0293 ± 0.0006) was found, independent of the oxidant or the metal ion catalyst. The mechanisms of oxidation of water are discussed in terms of the isotopic fractionation data.