Insoluble CS2.5H0.5PW12O40 heteropolyacid supported on Pd/MCF (CsPW/Pd/MCF-X (X= 10.0, 21.7, 35.7, 52.6, and 68.9)) was prepared with a variation of Cs2.5H0.5PW12O40 content (X, wt%) for use in direct synthesis of hydrogen peroxide from hydrogen and oxygen. High catalytic performance of CsPW/Pd/MCF-X catalysts compared to Pd/MCF catalyst was attributed to the enhanced acid property of CsPW/Pd/MCF-X catalysts. Selectivity for hydrogen peroxide and yield for hydrogen peroxide over CsPW/Pd/MCF-X catalysts showed a volcano-shaped curve with respect to Cs2.5H0.5PW12O40 content. Acidity of the catalysts also showed a volcano-shaped trend with respect to Cs2.5H0.5PW12O40 content. It was revealed that yield for hydrogen peroxide increased with increasing acidity of CsPW/Pd/MCF-X catalyst. Acidity of CsPW/Pd/MCF-X catalyst played a crucial role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. (C) 2010 Elsevier B.V. All rights reserved.