A hydrophobic peptide, Boc-(Ala-Aib)8-OMe (BA16M), and its end-modified derivatives were synthesized, and the pressure-area (pi-A) isotherms of the peptides spread at the air/water interface were studied from the view point of interhelix interactions. All pi-A isotherms of the synthetic peptides showed an inflection and weak irregular bumping at a surface areas of about 240 and 230 angstrom2/molecule, respectively, indicating that the helix axis of the peptide is oriented parallel to the interface. A small mound was observed at around 300 angstrom2/molecule in the pi-a isotherm of BA16M, which was ascribed to the phase transition from a liquid to a solid state. The monolayer of an equimolar mixture of the peptides having an opposite kind of charge in the end group underwent the phase transition in the pi-A isotherm, which was not observed with one of the two peptides. The electrostatic interaction between the end groups should stabilize the molecular packing at the interface.