Oxidation of cycloolefins by hydrogen peroxide catalyzed by various Keggin-type heteropolyacids, H 3 Pmo 12 - x W x O 40 , x=0-12, were studied in homogeneous tri-n-butyl phosphate solution, placing stress on the effects of the constituent elements of heteropoly anion. Remarkable synergistic effect of mixed addenda atoms, Mo and W, was observed and high yields of dialdehyde were obtained. A reaction scheme was proposed based on the time course and stoichiometry of reaction, and the reactivity of products.