Interaction of hydrophobized surfaces with mixtures of water and organic solvents is important in many practical applications. Alkylammonium clay minerals are excellent models for studying such processes. An example is the adsorption of n-butanol-water on vermiculite primed with dodecyl-, octadedcyl- and dodecyldiammonium ions. The thermodynamic functions Δ 21 G S, Γ 21 H S, Δ 21 S S for the adsorption of butanol from water at the interfaces were obtained from the reduced specific surface excess values and the calorimetric heat of displacement. They reveal the strong influence of the alkyl chain length. Two contributions are important: the enthalpy change due to the rearrangement of the alkyl chains from flat to paraffine-type orientation and the gain of entropy due to conformational changes of the chains.