Morphology and structure transformation of tungstic acid hydrate, H(2)WO(4)(.)nH(2)O precipitates, have been studied in aqueous phase with various levels of residual [Na+ ]s at room temperature and at 120 degreesC in autoclave. Spindle-shaped morphology is characteristic for the washing product of the freshly prepared precipitates. Residual sodium is a persistent contaminant in (H2WO4H2O)-H-.. Sodium ion content in the solid phase has been found to control reaction route at hydrothermal treatment of tungstic acid hydrates. Morphological changes against [Na (+)]s content at 120 degreesC are significant. Rectangular crystallites of WO3 H2O, hexagonal platelets of WO(3)(.)1/3H(2)O or their mixture can develop from the precursor spindle-shaped (H2WO4H2O)-H-. particles with various [Na+](s) at 120degreesC. It is noteworthy that particles that ripened at room temperature display faces and axes seemingly corresponding to those of the crystallites developed in the hydrothermal process.