The surface properties of illites and kaolinites of various origins and formation conditions were measured by inverse gas chromatography (IGC) at infinite dilution conditions. The value of the dispersive component of the surface energy (gamma(s)(d)) determined by IGC is significantly higher for these clays than the gamma(s)(d) of amorsphous silicas. This is explained by the partial insertion of a very small amount of injected alkane probes into structural defects located on the border layers of the clays. The use of branched bulky alkane probes allows the evaluation of the clay’s nanorugosity through the determination of a morphological index (I-M). The relation between gamma(s)(d) and surface nanorugosity (I-M) confirms that the high values of gamma(s)(d) of the clays are representative only of these particular sites of adsorption.