A floating molecular film was formed when a chloroform solution of dimethyldioctadecylammonium bromide (DMDOA(+)Br(-)) was spread on an aqueous dispersion of a clay mineral (sodium montmorillonite). At a low concentration (<50 ppm: ppm = mg dm(-3)), a clay mineral was exfoliated into negatively charged layers (denoted by clay nanosheets). Clay nanosheets in a dispersion were adsorbed onto a floating film because of electrostatic interactions. At various clay concentrations (0-50 ppm), surface potential was measured as a function of time to obtain the quantitative information about the adsorption of clay nanosheets on a condensed floating film of DMDOA(+) ions. It was concluded that the adsorption equilibrium obeyed the Langmuir adsorption isotherm, which was supported by the atomic force microscopy (AFM) observation. The rate constants of adsorption and desorption processes were determined by the fitting analyses based on the Langmuir type kinetics. Interestingly, the delay of the adsorption was observed in the early stage indicating that clay nanosheets were removed from the surface region through the repulsion by a counteranion (Br-). This explanation was supported by the model simulation using the forward difference method.