The surface properties of six clay samples (three illites and three kaolinites formed under different logical conditions) were evaluated by inverse gas chromatography at finite concentration conditions. The determination of the adsorption isotherms for several organic probes, hexane, benzene, pyridine, and 2-propanol, on these clays allows the estimation of their energetic surface heterogeneity through the determination of their adsorption energy distribution functions. It was then shown that, when the organic probe is well chosen, this method allows distinguishing between the family of clay, illite or kaolinite, and even, in a same family, between the different geological formation conditions. The illite samples are more heterogeneous than the kaolinite samples.