Taking advantage of the fact that H3PMo12O40 is highly soluble in some organic solvents, membrane-like H3PMo12O40-blended polysuIfone film and H3PMo12O40-blended polyethersulfone film were prepared by using dimethylformamide as a solvent for both. The catalytic activity of H3PMo12O40 was remarkably enhanced by blending it with a polymer to form a porous film where H3PMo12O40 is uniformly and finely dispersed. The H3PMo12O40-blended polysulfone film showed a higher activity for the oxidation reaction (reaction to acetaldehyde) and a lower activity for the acid catalyzed reaction (reaction to ethylene or diethylether) than H3PMo12O40. The H3PMo12O40-blended polyethersulfone film showed higher activities for the oxidation reaction and for the acid catalyzed reaction than H3PMo12O40. The oxidation activity in two film catalysts was about IO times higher than unsupported H3PMo12O40.