Hydrous tungsten oxide platelet particle was precipitated from acidic solution of an ammonium tungstate at 80 degrees C. By dissolving the tungstate salt in nitric acid at low temperature before being brought up to elevated temperature, the precipitating products were obtained at very high yield compared to direct mixing of the two reactants at elevated temperature. The yield increased rapidly with an increase of nitric concentration and became constant at moderate acid concentration. Higher yield was obtained from the more concentrated tungstate solution. The as-precipitated powders consisted of mixed hydrotungstite (H2WO4 center dot H2O) and tungstite (WO3 center dot H2O). Electron microscopy study on the as-precipitated powder clearly revealed very fine platelet particle having average size of 60 +/- 5-nm wide and 15 +/- 3-nm thick loosely agglomerated to a size of 300-500 nm. A WO3 thick film sensor made from the precipitate calcined at 500 degrees C consisting of 90 +/- 10-nm wide and 30 +/- 5-nm thick platelet particle was highly sensitive to 3-10 ppm ethylene gas at operating temperature of 300 degrees C. (C) 2008 Elsevier B.V. All rights reserved.