In order to throw light on the mechanism of organic compounds-clay interactions and to control the preparation of hydrophobic fillers, the effect of asphaltene adsorption from water-saturated toluene onto various clay minerals (kaolinite, illite) was studied. The modification of the clay surface charge and wettability were characterized, respectively, by microelectrophoresis and Washburn contact angle measurements. Further, the alteration of the surface area and chemical composition of the clay particles due to asphaltene adsorption were assessed using various methods such as N-2-adsorption/desorption, X-ray diffraction (XRD) measurements, FT-IR and scanning electronic microscopy. The data indicate that upon asphaltene adsorption, the clay particles become hydrophobic, their contact angle with water increases, while their surface charge and surface energy are reduced. Furthermore, according to XRD data, no swelling of the clay by asphaltenes occurs in the water-saturated toluene, indicating that asphaltenes cover the clay's external surface and lead to their aggregation, either in water or in the vacuum dried state. Finally, we elucidate the mechanisms by which asphaltenes adsorb and modify the clay surface characteristics. Such mechanisms involve mainly interactions that occur between the asphaltenes' surface groups and silanol/aluminol groups of the mineral. (C) Koninklijke Brill NV, Leiden, 2009