Abstract The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA] 4 [γ-SiW 10 O 34 (H 2 O) 2](I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30% aqueous hydrogen peroxide. The negative Hammett ρ+(− 0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X SO= 0.04, for I-catalyzed oxidation of ...