Asphaltene and water adsorption isotherms on clay surfaces have been determined to approach the hydrophilic/hydrophobic character of several samples of kaolinites and illites. It is shown that the nature and the genesis condition of the clays affect the adsorption of both asphaltene and water. A larger amount of asphaltene is adsorbed on kaolinites while illites show more affinity for water. Cation hydration is shown to be responsible for part of the total hydrophilicity of the clays. It is also found that 25% of the kaolinite surface is hydrophilic, whereas this value extends to 40% for illite. Finally, it is shown that hydrophilicity decreases, whereas asphaltene adsorption increases.