The miscibility of dipalmitoylphosphatidylcholine and oleic acid (DPPC/OA) and of dipalmitoylphosphatidylcholine/oleic acid/cholesterol (DPPC/OA/Chol) in mixed mono-layers was determined. According to the thermodynamic calculations, the energy involved in the process is very low, suggesting the lack of strong interactions. Moreover, the presence of oleic acid or cholesterol has little influence on the transition temperature of DPPC.