By doping of polyaniline (PANI) in PdCl2 aqueous and ethanol solutions the catalysts containing crystalline and colloidal Pd particles of different sizes were prepared. The size of palladium particles present in Pd/PANI catalysts (characterised by SEM and XRD methods) influenced the course of 2-ethylanthraquinone (eAQ) hydrogenation, a key step in the industrial production of H2O2. The presence of large palladium particles promotes reactions leading to the formation of the so-termed "degradation products" not capable of hydrogen peroxide formation.