We have studied the formation of ultrathin hybrid films composed of a water-soluble alkylammonium cation (trimethyloctadecylammonium. caiton: TMODAH(+)) and clay nanosheets at an air-water interface. When a chloroform solution of the ammonium salt of TMODAH(+)Cl(-) was spread onto a surface of an aqueous clay suspension in a Langmuir trough, a stable monolayer of the water-soluble ammonium cation was formed by hybridization with the clay nanosheets at the air-water interface. Surface pressure-molecular area ( T-A) isotherm curve of the hybrid monolayer shifted to the smaller molecular area side with the increase in the concentration of the ammonium salt in the solution or with the decrease in that of the clay in the suspension. In some cases, the lift-off areas in the π-A isotherm curves were less than the cross-sectional area of an alkyl chain (ca. 0.19 nm(2)). These results indicated that some ammonium cations spread onto the clay suspension were dissolved into the aqueous subphase before the hybridization with the clay nanosheets. The hybrid monolayers were transferred onto a glass plate in a layer-by-layer way by horizontal dipping to form a hybrid multilayerd film. Interestingly, the densities of TMODAH(+) determined by the infrared spectral analysis were constant in the hybrid multilayers prepared from the clay suspensions at the same concentration, regardless of the concentrations of the ammonium salt solutions. XRD patterns of the films showed that the cations of TMODAH(+) would lie down on the clay layer in the hybrid film.