The use of infrared spectroscopy as a technique for characterizing the state of intercalation and exfoliation in polymer nanocomposites prepared from montmorillonite-based nanoclays was investigated. The nanocomposite samples were based on polypropylene (blown films) or high-density polyethylene (extruded material). It was clearly shown that the shape of the clay band envelope in the 1350-750 cm(-1) region, which includes four Si-O stretching modes, varies with the degree of processing and is sensitive to the quality of intercalation/exfoliation. Peak fitting was used to elucidate the nature of the changes and to develop quantitative indicators. The out-of-plane Si-O mode near 1070 cm(-1) is particularly sensitive and undergoes significant changes. Infrared spectroscopy is a valuable complement to established techniques like X-ray diffraction and transmission electron microscopy and has the added advantage of being able to provide a relatively fast indication of the overall degree of intercalation/exfoliation, including clay particles with interlayer spacings outside the range of X-ray diffraction. Furthermore, it offers the possibility of use as a quality control method, either in the laboratory or on-line.