An acidic resin bearing SO3H functional groups within its reticular structure acts as an efficient support for the in situ formation of highly active Pd nanoparticles (NPs) responsible for the direct synthesis of hydrogen peroxide from H2 and O2. Characterization by means of X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM) reveals that not only the acidity of the resins but also the mean size of the Pd NPs are crucial factors in achieving efficient catalytic performance. The acidic resins act effectively as supports, while basic resins hardly activate the reaction and do not induce any structural change around the Pd center. [on SciFinder(R)]