Heteropolyacid (H3PW12O40, HPA)/SiO2 nanoparticles were synthesized by a microemulsion technique to prevent the dissolution of HPA in polar solvents including water. The particle size was controlled within the range from 100 to 500 run. The solid-state NMR, FT-IR and the adsorption of trimethylphosphine were used to characterize the HPA/SiO2 nanoparticles. The surface properties including the BET surface area, acidity and catalytic activity depended on the particle size of HPA/SiO2. The reduction of particle size of HPA/SiO2 improves the surface area and the acidity of the nanoparticles. The interaction of HPA with silica surfaces was observed by solid state NMR. Selective ethanol oxidation was tested as probe reaction on HPA/SiO2. The catalytic activity of ethanol oxidation was enhanced with the reduction of the particle size of HPA/SiO2. The catalytic properties of HPA in the silica nano phase might be different from those in the bulk phase due to the physico-chemical differences of nano phase. (c) 2005 Elsevier B.V. All rights reserved.