The interaction of multivalent rare earth cations with clay minerals is of importance for the design of new materials based on layer silicates. The effect of thermal and hydrothermal treatments upon the local structure around lutetium ions intercalated within the interlamellar space of montmorillonite has been studied by X-ray absorption spectroscopy (EXAFS). The initial Lu(III) hydrate (7.7 Lu-O at 2.30 angstroms), formed after the ion exchange process, loses one water molecule after air-heating at 300[degree]C (6.5 Lu-O at 2.29 angstroms) and is fully disrupted by air heating at 500[degree]C. After this treatment oxide- and hydroxide-type environments are detected, characterized by higher shells (Lu-Lu at 3.3 angstroms and Lu-O at 4.1 angstroms) and distortions in the first Lu-O contribution (5.9 Lu-O at 2.22 angstroms). At 700[degree]C the transformation concerns all lutetium atoms that now show the oxide structure (5.4 Lu-O at 2.22 angstroms and 2.8 Lu-Lu at 3.33 angstroms). The spectra recorded after thermal treatment at higher water vapor pressure (100 atm) can be well fitted with on three shells corresponding to the disilicate phase. 30 refs., 11 figs., 3 tabs.