The dehydration of tungstic acid hydrate, H2WO4.H2O samples prepared by the methods of Freedman [J. Am. Chem. Soc. 81 (1959) 3834.] ([Na](residual) less than or equal to 10 ppm) and Zocher and Jacobson [Kolloidchem. Beih. 28-6 (193,9) 167.] ([Na](residual) = 140 ppm) has been studied by in situ high-temperature XRD, thermogravimetry and FTIR at room temperature. The formation of cubic tungsten oxide, c-WO3 has been observed in the course of the dehydration at the loss of the second water molecule: H2WO4 --> c-WO3. Keeping the temperature constant at 300 degreesC under air, the metastable c-WO3 transforms into the stable orthorhombic, o-WO3 phase. FTIR and TG results have shown that the 300 degreesC dehydration products contained hydrogen in the form of OH groups and bronze-like W-O-H even after the loss of the second water molecule. A H2WO4.H2O sample embedded in silicone grease has also been studied by high-temperature XRD, TG and FTIR. The c-WO3 --> o-WO3 conversion was blocked in this sample, and FTIR has indicated the presence of CO vibrations in the transmittance spectrum. Lattice parameters of c-WO3 samples produced from H2WO4.H2O (this work), and results from the literature (from other precursors) have been compared with the conclusion, that the formation of closely monophase c-WO3 and the relative stability of this phase are in correlation with the presence of some kind of impurity (originating from silicone grease in this experiment and from various organic or inorganic precursors applied at other laboratories).