Extremely small Pd nanoparticles have been successfully obtained from reduction of Pd(OAc)2 precursor inside two highly cross-linked polymers, namely a poly(ethylstyrene) and a poly(4-vinylpyridine). The structural, optical, and vibrational properties of the Pd/polymer composites have been fully characterized by several techniques (FT-IR, UV−vis, and XAS spectroscopies, coupled with TEM microscopy) during all the synthetic steps. It is demonstrated that the nature of the polymeric matrix has a strong influence on the formation of the Pd nanoparticles and affect their final properties, in terms of particle size, electronic properties, and type and fraction of accessible surface sites. In particular, the presence of nitrogen-based ligands is at the base of the formation of extremely small, subnanometric Pd clusters in P4VP, as demonstrated by the observation of a specific interaction between Pd nanoparticles and the nitrogen of the pyridine ligands.