The IR spectra of the pyridine has been measured following adsorption on and subsequent desorption from Fe-montmorillonite (Fe-mont.). The exposure of strong Brnsted and Lewis acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ IR spectroscopy of small adsorbed base molecules, often pyridine (Py), CO, NH3 or NO molecules have been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room and higher temperature regimes. Since it is more selective, stable and more strongly adsorbed than others. Three modes of adsorption have been observed, (a) electron transfer at Lewis acidic surface sites, (b) proton transfer at Bronsted acidic surface sites and (c) hydrogen bonding to surface hydroxy groups. In this study, important correlations and schifts adsorption of pyridine were discussed. In conclusion, FT-IR spectroscopy was used successfully with pyridine as a basic probe, to determine the Bronsted and Lewis acid sites on the surface of the Fe-montmorillonite. The Fe-montmorillonite has both Lewis and Bronsted acidity. (c) 2005 Elsevier B.V. All rights reserved.