A three-phase system with a chemically heterogeneous solid/liquid interface and a homogeneous liquid/gas interface is built up by means of the Langmuir-Blodgett technique. The miscibilities of two cationic surfactants with the same hydrocarbon chain length but different chemical structure and dimension head groups are studied to form the heterogeneous monolayers. The application of the miscibility criterion reveals partial phase separation of the two components. Static contact angles are determined after the downward LB deposition of a first heterogeneous monolayer, and after the upward deposition of a second homogeneous monolayer. In the latter case, resembling wetting of a chemically heterogeneous solid by monocomponent liquid, an independence of the receding static contact angles on the heterogeneous composition is found. The insensitivity of wetting to the composition may result from the intermolecular interactions established for the partially immiscible monolayers. These interactions probably quench the difference in the properties of the heterogeneous mixtures at the high surface pressure at which the LB system is formed.