Co(II) complexes and Co(H2O)62+ are used as catalysts in advanced oxidation processes. Therefore it was decided to study the kinetics of reaction of Co(H2O)62+ with H2O2. Surprisingly, the kinetic results point out that the process involves three consecutive reactions, each of them requiring an H 2O2 molecule, i.e. three H2O2 molecules ligate to the central cobalt cation prior to the formation of radicals. DFT analysis suggests that the transient (H2O) 3CoII(OOH)2(H2O2) decomposes via: (H2O)3CoII(OOH) 2(H2O2) → (H2O) 3CoII(OOH)(̇OOH)(OH) + ̇OH ΔG0 = -5.975 kcal mol-1, with no evidence for the formation of a Co(iii) transient. It is proposed that analogous mechanisms are involved whenever the redox potential of the central cation is too high to enable the reaction: M(H2O)6n+ + H 2O2 → M(n+1)+aq + ̇OH + OH-.