The thin film of an ion-exchanged adduct of a clay and tetra-n-decylammonium cations ((n-decyl)(4)N+) was formed at an air/water interface. A clay-modified electrode was prepared by depositing the film onto a glassy carbon substrate with the Langmuir-Blodgett method. When the electrode was soaked in an aqueous solution of hexa-ammineruthenium(III) chloride ([Ru(NH3)(6)]Cl-3), (n-decyl)(4)N+ cations were replaced with [Ru(NH3)(6)](3+) ions. The electrochemical behavior of [Ru(NH3)(6)](3+) in the ultra-thin film was studied by measuring the cyclic voltammetry and coulometry on the clay-modified electrode, The atomic force microscope images of a saponite film deposited on a microglass plate and saponite-(n-decyl)(4)N+ transferred on a microglass plate were compared.