Influence of thermal pretreatments (under N-2, air or H-2 gas atmosphere at 500 degrees C or 700 degrees C) has been investigated for the Pd/Al2O3 and Pd/Carbon catalysts in terms of its effect on their Pd particle size and performance in the H-2-to-H2O2 oxidation and H2O2 destruction (by decomposition and/or hydrogenation) reactions in aqueous acidic medium containing Br promoter. The influence on the net H2O2 formation is found to depend strongly upon the catalyst support due to support-Pd cluster interactions. For both the catalysts, the thermal treatments (except in air) caused a large increase in their Pd particle size. The increase in Pd particle size caused an increase in the H2O2 formation activity of Pd/Al2O3 but a decrease in the H2O2 formation activity of Pd/C.