Crystalline H2WO4.H2O, (WO3.2H(2)O) powder samples have been prepared by acidification of alkali tungstate aqueous solution. In order to decrease the alkali ion content of the precipitate from about 6 x 10(5) to 10-10(2) ppm, wash treatments have been applied. The influence of the wash treatment on the structure and hydration state of the solid product has been investigated by X-ray powder diffraction (XRD) and infrared absorption spectroscopy (IR). Morphology of the precipitate and the time development of the morphology during washing steps have been studied by scanning electron microscopy (SEM). The formation and transformation of an intermediate, the so-called tungstic acid C phase have been observed in the sequence of the washing steps. Reversible changes in the hydration state: WO3.2H(2)O<->WO3.H2O have also been observed at washing with long time interaction between crystallites and washing liquid with pH greater than or equal to 4.6.