Oxidation of cycloolefins (cyclohexene, cyclooctene, and cyclododecene) with a 30% solution of hydrogen peroxide at 65 degreesC in the presence of heteropoly acids (HPA) H3PW12-xMoxO40 (x = 0-12), which are precursors of active peroxo complexes, and phase transfer catalysts Q(+)Cl(-), where Q(+) is the quaternary ammonium cation containing C-4-C-18 alkyl groups or [C5H5NC16H33](+), was studied. The catalytic activity decreases in the HPA series: H3PW12O40 > H3PW9Mo3O40 > H3PW6Mo6O40 > H3PW3Mo9O40 > H3PMo12O40. The state of the H3PW12O40-H2O2 system was studied using UV, IR, and P-31 NMR spectroscopies with variation of the [H2O2] : [HPA] ratio from 2 to 200 during cyclohexene epoxidation. Despite different catalytic precursors, the reaction proceeds through the same peroxo complex.