A solution of the water-insoluble quaternary alkylammonium salt with the long hydrocarbon chain, CH3(CH2)29N(CH3)3-I (C(30)TMA(+)I(_)), was spread on water and clay mineral (montmorillonite) dispersion surfaces to investigate film formation behavior of the amphiphilie ammonium cation by pi-A isotherm curve measurements, AFM observation, IR spectroscopy, and XRD measurements. The C(30)TMA(+)cations formed a stable Langmuir film on the water surface. The Langmuir films were deposited on a substrate to fabricate the multilayer, in which the cations stood nearly perpendicularly to the substrate with the all-trans conformation and they were packed closely with two-dimensional ordering. The negatively charged clay-mineral-layers were adsorbed on a floating Langmuir film of the cation at an air-clay mineral dispersion interface. On the dilute clay mineral dispersion surface, some parts of the floating film of the cation were adsorbed by the clay-mineral-layers and the C(30)TMA(+)cations stood on the clay-mineral-layers. As the clay mineral concentration in the dispersion increased, the pi-A isotherm curve for the C30TMA(-)cation at the air-dispersion interface shifted to the larger molecular area side with high collapse pressures (40-50 mN m(-1)). Almost whole area of the floating film was covered with the clay-mineral-layers on the concentrated dispersion surface, and the cations stood nearly perpendicularly on the clay-mineral-layers.