Palladium catalysts supported on SO3H-functionalized SiO2 and TiO2 (denoted as Pd/SO3H-SiO2 and Pd/SO3H-TiO2) were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. For comparison, palladium catalysts supported on bare SiO2 and TiO2 (denoted as Pd/SiO2 and Pd/TiO2) were also employed to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Selectivity and yield for hydrogen peroxide over Pd/SO3H-SiO2 and Pd/SO3H-TiO2 catalysts were higher than those over Pd/SiO2 and Pd/TiO2 catalysts. SO3H-functionalized SiO2 and TiO2 supports efficiently served as an alternate acid source in the direct synthesis of hydrogen peroxide from hydrogen and oxygen.