The catalytically important, two-dimensional vanadyl hydrogen phosphate hemihydrate, VOHPO4·0.5H2O phase was transformed to the one-dimensional vanadyl hydrogen phosphate tetrahydrate, VOHPO4·4H2O for the first time by mild treatment with water vapor. It was also observed that for three samples of VOHPO4·0.5H2O prepared under different conditions, the rate of transformation to the tetrahydrate was markedly affected by morphology and crystallinity. The tetrahydrate phase could also be re-transformed to the hemihydrate by applying high vacuum or by heating at 120 °C, but this was accompanied by significant loss in crystallinity.
