The dehydration mechanism of 12-tungstophosphoric acid H3PW12O40 and of its acidic cesium salts CsxH3-xPW12O40, have been investigated by in situ FTIR spectroscopy at 240 K. Both protonic species and vibrational modes of the Keggin anions have been followed in situ as a function of the extent of dehydration. After being outgassed in vacuo at ambient temperature, di-aqua protons, H5O2+, and/or hydroxonium ions, H3O+, species were detected for H3PW12O40 and Cs2HPW12O40 samples. These species are characterized by a delta (H2O) vibration band at 1708 cm(-1) and a v(OH) vibration band at 3355 cm(-1). The intensity of these absorption bands progressively decreased as the protons were substituted by Cs+. Further dehydration leads to the decomposition of the former protonic species with the formation of hydroxyl groups, characterized by a broad absorption band centered at similar to 3209 cm(-1). After degassing at 673 K, the heteropolycompounds were completely dehydroxylated. The decomposition of protonic water clusters induced strong modifications of the vibration bands of the Keggin anion ascribed to changes in bond lengths and in the local symmetry of the anion. The extent of these modifications is closely connected to the proton content of the heteropolyacid. The Cs2.7H0.3PW12O40 sample showed only minor changes with dehydration. (C) 2001 Academic Press.