The effect of the number of protons in the Ti(IV)-monosubstituted Keggin polyoxometalate Na5-nHnPTiW11O40 (n= 1-5; Ti-POM) on its catalytic behaviour in cyclohexene (CyH) oxidation with aqueous H2O2 in MeCN is reported. It has been found that Ti-POMs with n = 2-5 catalyse efficiently CyH oxidation to yield trans-cyclohexane-1,2-diol as the main reaction product, while Ti-POM containing only one proton shows lower activity in CyH oxidation and produces allylic oxidation products, 2-cyclohexene-1-ol and 2-cyclohexene-1-one, along with comparable amounts of the corresponding epoxide and diol. The obtained results strongly support homolytic oxidation mechanism for CyH oxidation in the presence of the monoprotonated Ti-POM and heterolytic oxygen-transfer mechanism in the presence of Ti-POMs having two and more protons. The P-31 and W-183 NMR studies revealed that Ti-POMs are stable towards at least 100-fold excess of H2O2 and the high catalytic activity of Ti-POMs with n = 2-5 is not due to the formation of lower nuclearity species. The addition of one equivalent of H+ to the monoprotonated peroxo complex [Bu4N](4)[HPTi(O-2,)W11O39] (I, (PNMR)-P-31 in MeCN: - 12.40 ppm) results in the formation of the diprotonated titanium peroxo species [H2PTi(O-2)W11O39](3-) (II, P-31 NMR in MeCN: - 12.14 ppm). This peroxo species can also be obtained by adding an excess of H2O2 to Na5-nHnPTiW11O40 (n = 2-5) in MeCN. The presence of the second proton in the peroxo species is a crucial factor determining the capability of II to oxidise alkenes via heterolytic oxygen transfer mechanism. Both P-31 NMR and GC-MS studies corroborated that II reacts with CyH producing trans-cyclohexane- 1,2-diol as the main reaction product, whereas I is not reactive towards CyH under stoichiometric conditions. (c) 2005 Elsevier B.V. All rights reserved.