A new facile method is proposed for the preparation of aqueous sols of highly crystalline tungsten trioxide with a particle size of 60-150 nm, containing no organic stabilizers/surfactants. These sols possess high sedimentation stability, which is quite unusual for inorganic colloidal systems containing relatively large particles, with a rather high density (rho (c)(WO3) = 7.3 g/cm(3)). The method is based on the thermal decomposition of ammonium paratungstate, followed by dispersing the resulting powders in water under ultrasonic treatment. Thermal decomposition of ammonium paratungstate, and the composition and structure of the resulting tungsten trioxide and its aqueous dispersions, were investigated with thermal analysis combined with the mass spectrometry of gaseous thermolysis products, powder X-ray diffraction, scanning electron microscopy, low-temperature nitrogen adsorption, IR spectroscopy and dynamic light scattering. It has been demonstrated that the high sedimentation stability of WO3 results from electrostatic stabilization, which might be caused by the formation of tungstic acid on the surface of WO3 particles when they come into contact with water. The nanocrystalline WO3 obtained can be used to produce gas sensors for ammonia.