We report the formation of a monolayer of perfluorostearic acid (PFS, CF3(CF2)(16)COOH) using a mixed solvent of pentafluorobenzene (PFB) and methanol (weight ratio of methano/PFB =similar to 1 : 86) as a PFS spreading solvent. PFS formed very stable monolayer at air/water interface, which has 0.32 nm(2) of molecular area and 63.4 mN/m of collapse pressure on DI water. The presence of all the metal cations such as Na+, Ca2+, Co2+, Cu2+ and La3+ results in the expansion of the monolayer, the surface pressure (pi)-area (A) curves of PFS on Ca2+ or La3+ subphase showing the most expanded isotherms. Polyion complex formation between PFS and water-soluble polymers was observed. Especially, polyethyleneimine (PEI) formed a strong polyion complex with PFS. Changing the pH of the subphase controlled the degree of the complex formation between PFS and the PEI. The mixed monolayers of PFS and stearic acid (SA) showed positive molecular area change on mixing due to repulsion between PFS domains and the SA domains on the subphase.