The thermal dehydration of the Ca(H2PO4)(2) . H2O-SiO2 system has been studied by a continuous thermogravimetric mode method, and by a batch method, heating the mixtures in the molar ratios SiO2/Ca(H2PO4)(2) . H2O = 0.5, 1, 2 and 3 at 150, 185, 245, 275, 320, 430 and 780 degrees C. The batches, after cooling, were examined by X-ray diffraction (XRD) and infrared spectroscopy (IR) to identify the intermediate and final products. The final products were mainly Ca(PO3)(2) and SiP2O7, while pyro- and triphosphates and Si(HPO4)(2) were detected as intermediate products. The quantity of SiO2 present affects the dehydration which takes place in five stages. The removal of water molecules in the first, second and fourth stages was increased, and in the third and fifth stages was decreased on increasing the amount of SiO2.