Epoxidation of hexene-1 and cyclohexene in the systems H2WO4/H2O2-H2O/H3PO4 was studied under the phase-transfer catalysis conditions depending on the ratio between the reactants Na2WO4 and H3PO4 (in the presence of excess H2O2) and pH of the water phase. The interrelation between the pH of the aqueous phase, the composition of phosphatooxoperoxotungstate ions formed, and the catalytic activity of the systems was established. The complexes Q(3)[PO4{WO(O-2)(2)}(4)] and Q(2)[HPO4{WO(O-2)(2)}(2)], where Q(+) is the lipophilic cation of a phase-transfer agent whose nature can have a noticeable effect on the composition of peroxoheteropoly compounds formed, exhibited the highest activity. To achieve the maximum catalytic activity, it is necessary to maintain the stoichiometric reactant ratio corresponding to the composition of peroxo complexes that prevail in a given pH range.