The H2O2-based epoxidation of bridged cyclic alkenes in a monophasic system containing low concentrations (<2 mM) of [(BU4N)-N-n](4)[(Pr2NH3)-N-i](2)H[P[Ti(O-2)}(2)W10O38] (.) H2O (1) (with two eta(2)-peroxotitanium sites in the anion) has been studied in search of the catalytically active species involved. P-31 NMR spectra of 1, measured under a variety of conditions, revealed that the active species was not hydroperoxotitanium complex [P{Ti(OOH)}(2)W10O38](7-) or [P{Ti(OOH)}Ti(O-2)W10O38](7-). The reaction pathways for the alkene epoxidation are discussed to understand the kinetics (especially the initial [H2O2] dependence). It was concluded that the net catalytic reaction for the epoxidation occurred through the two-electron oxidation at the hydroperoxotitanium site in the catalyst. (C) 2002 Elsevier Science B.V. All rights reserved.