The oxidation state and the structural properties of Al2O3-supported bimetallic PdCu catalysts during the catalytic reduction of KNO3 carried out in the aqueous phase were investigated by X-ray absorption spectroscopy. Under reaction conditions the noble metal component (Pd) was in a reduced state, while the less noble metal (Cu) was found to be partially oxidized. A PdCu phase was formed in the bimetallic catalysts, which appears to be located in small domains on the surface of Pd rich particles.