The room temperature ageing process of tungsten oxide dihydrate grains (H2WO4.H2O) prepared by the methods of Zocher and Jacobson (Kolloidchem. Beih. 28 (1929) 167) and Freedman (J. Am. Chem. Soc. 81 (1959) 3834) has been studied. A comparison between these two preparation methods and the ageing characteristics is reported. Zocher and Jacobson-type grains, spindle like in shape, have a strong tendency to morphological conversion depending on the pH value of the surrounding aqueous solution. Under the same conditions the Freedman-type irregularly shaped particles preserve their morphology. Scanning electron microscopy and Fourier transform infrared spectroscopy studies have shown that this structural and morphological stability of Freedman-type grains can be altered by the presence of alkaline (Li+, Na+, K+) and NH4+ ions. A limited incorporation of cations into the tungstate structure was observed. (C) 2002 Elsevier Science B.V. All rights reserved.