The polyoxometalate particles with Keggin structure, (NH 4) 3PMo 12 O 40 ·9H 2O(1) and (NH 4) 3PW 12 O 40 ·7H 2O(2), were prepared firstly by solid state reaction at room temperature. Their structure, particle sizes and appearances were characterized with elemental analyses, IR, XRD and TEM techniques. The size distribution of nanoparticle 1 is in the range of 5—14 nm, the mean size is 9 nm, and the size distribution of nanoparticle 2 is in the range of 3—12 nm, the mean size is 6 nm. It was found that the product H 2C 2O 4·2H 2O plays a critical role in preventing the nanoparticles from aggregating to large particles.
