The dehydration of H4SiW12O40. 15.6H(2)O was studied in situ in the IR chamber. On evacuation at room temperature the departure of most loosely bonded water characterized by bands at 3550 and 1616 cm(-1) was observed. In the remaining hexahydrate the band at 3445 cm(-1) was ascribed to the hydrogen bond between the O-d oxygen atom of the Keggin unit and dioxonium H5O2+ ion, the presence of which is manifested by the 1710 and 1100 cm(-1) vibrations. All these bands vanish in the case of anhydrous H4SiW12O40, in which the band at 3106 cm(-1) ascribed to the hydrogen bond between neighbouring HPA anions O-d-H+-O-c is still present. The dehydration of hexahydrate is accompanied by splitting of the W=O-d band into 987 and 1010 cm(-1) reflecting the change of the kind of hydrogen bond in which the O-d oxygen atom is involved. Based on the above results it was concluded that protons forming oxonium ions in hydrated solid heteropoly acid are more strongly bonded than those in anhydrous one which are forming hydrogen bonds between neighbouring Keggin units.