The structural and chemical stabilities of immobilized polyoxometalate (POM)-containing catalysts are crucial factors for their industrial application. An alumina supported POM catalyst is prepared by using a facile condensation reaction between the trilacunary POM Na₁₂[α-P₂W₁₅O₅₆]⋅24 H₂O (P₂W₁₅) and the hydroxy groups on the surface of γ-Al₂O₃ spheres under acidic conditions. The heterogeneous catalyst P₂W₁₅−Al₂O₃ is characterized by a wide variety of techniques and shows excellent stability and highly efficient reactivity and selectivity for the oxygenation of thioethers to sulfoxides, which are a very useful intermediate in organic synthesis and the industrial preparation of drugs. Furthermore, P₂W₁₅−Al₂O₃ can be recycled and reused at least ten times without any observable loss of its catalytic efficiency, mainly due to the covalent immobilization and high dispersion of P₂W₁₅ on the γ-Al₂O₃ surface.