Inverse gas chromatography, IGC, has been used to investigate the surface properties of a kaolinite clay. Changes in enthalpies of adsorption for a range of probes and in the surface energies of the clays have been measured and the effect of calcination of the native clay as well as of coating with an aminopropylsilane coupling agent have been determined. The surface energy of the clay was lowered by calcination and further considerably reduced on coating with the silane. From the retention of polar probes, information on the accessibility of surface sites to the probes and on the acid-base character of the surface was measured. Two commonly used methods for quantifying these specific interactions are compared and yield similar results. The hydrated clay became less porous and less acidic after calcination while coating with the silane conferred a largely, though not exclusively, basic character.