The influence of thallium addition on the catalytic properties of palladium catalysts supported on SiO2 in the reaction of aldoses (glucose and lactose) oxidation to aldonic acid was studied. It was found that thallium does not have any activity by itself, but acts as "promoter" for the catalytic activity and selectivity of monometallic Pd/SiO2 catalysts. The XRD studies proved the presence of intermetallic compounds, which probably increases activity and selectivity of bimetallic catalysts in those reactions. The same interaction was found using ToF-SIMS. The TPO studies show that palladium oxidation occurs at much higher temperatures than in the case of thallium. A reaction involving bimetallic catalysts in oxidizing atmosphere at 333 K should not lead to surface oxidation of palladium and thus their deactivation. (c) 2005 Elsevier B.V. All rights reserved.