The surface free energy and surface polarity of organically modified montmorillonite clay change when exposed to elevated temperatures. This was verified in this study using contact angle measurements and Wu's harmonic-mean equation. It was observed that the surface polarity of organically modified clay measured from the values of contact angle of water and diiodomethane reduced significantly owing to thermal degradation of the organic modifier of clay, the latter was confirmed by Fourier transform infra-red spectroscopy. As a consequence, nonpolar polymer, polypropylene (PP), was found to spread more on heat treated surface of organically modified clay than a polar polymer, polyamide 6 (PA6). The ramification of these phenomena on blending of PP, PA6, and organically modified clay is discussed. (c) 2005 Published by Elsevier Ltd.