Inverse gas chromatography (IGC) has been used to investigate the surface properties of calcined kaolins and the effect on the mechanical properties of a Nylon-6 composite containing the clay. Modification of the kaolin with an aminosilane coupling agent lowered its surface energy so that it was comparable with that of the polymer matrix and changed the surface from being acidic to being predominantly basic. The surface of the nylon was also found to be basic but susceptible to hydrogen bonding. Incorporation of the kaolin into nylon changed the mechanical properties and raised the modulus and toughness of the materials. Better results were achieved for the coated kaolin than the uncoated material and this correlated with the surface energy measurements. (C) 2004 Elsevier Ltd. All rights reserved.