The reactions occurring by the clay framework during the thermal treatment of montmorillonite, beidellite, saponite, hectorite and Laponite saturated with the Keggin ion [(AlO4Al12VI)-O-IV(OH)(24)(H2O)(12)](7+) were investigated by gradual heating KBr disks of the clay samples up to 600 degrees C and recording their FTIR spectra at each 50 degrees increment. During the thermal treatment the Keggin ion undergoes dehydration, dehydroxylation and rearrangements, the latter is seen from changes in the Al-IV-O and Al-VI-O vibrations. From 500 degrees C clays dehydroxylate and are transformed into new phases with some silanol groups on the surfaces of the tetrahedral sheets. This reaction occurs at lower temperatures in the Keggin-treated smectites compared with Li- or Na-exchanged smectites. At 550 degrees C, there is a condensation reaction of the dehydroxylated Keggin cation with Si-O-groups on the surface of the tetrahedral sheet, giving rise to new bands at 614 and 402 cm(-1). This condensation reaction increases considerably at 600 degrees C. The framework of the heated pillared clay preserves the layer structure of the precursor smectite. The adsorption of water by the pillared clays becomes very intense after heating the disks to 600 degrees C.