Isothermal and non-isothermal transformations of Mg(H2PO4)(2) . 4H(2)O in vacuum, water vapour, dry and humid air atmosphere at 20-900 degrees C were studied by the methods of DTA, XRD, paper chromatographic analyses, gravimetry and optical microscopy. It has been shown that the size of initial tetrahydrate crystals and the composition of gaseous phase affect not only the dehydration rate but the chemical and phase composition of the resulting intermediates as well. At the same time, the crystalline polyphosphate [Mg(PO3)(2)](n) is the end product, irrespective of the heat-treatment conditions.