Uniform spherical colloidal particles of zirconium and thorium heteropoly tungstosilicate compounds were produced by aging aqueous solutions of salts of the corresponding metals and the heteropoly acid H4SiW12O40 at elevated temperatures. While the chemical analysis showed the thorium ion to substitute for four protons, the zirconium compound was more complex, because hydrolyzed cationic zirconium species reacted with the heteropoly anion to yield solids of nonstoichiometric composition. Electrophoretic mobility measurements showed their isoelectric points to be at pH similar to 3. In both cases, the original particles were amorphous, but the IR analysis indicated that the heteropoly anion retained the Keggin structure. On calcination at >750 degrees C both compounds crystallized into a mixture of oxides, but the particles remained spherical.