An ion-exchange adduct of saponite with tetra-n-decylammonium cation ((n-decyl) 4 N (super +) ) was prepared and dispersed in chloroform. The material was spread on a water surface to form a thin film at the air-water interface. From the measurements of surface pressure vs. area, it was concluded that the film consisted of the very thin layer which was a mixture of single and double layers of saponite adduct. A clay-modified electrode was prepared by depositing the film on a glassy carbon substrate using the Langmuir-Blodgett method. When the electrode was soaked in an aqueous solution of [Fe(phen) 3 ](ClO 4 ) 2 (phen = 1,10-phenanthroline), (n-decyl) 4 N (super +) cations were replaced by [Fe(phen) 3 ] (super 2+) ions from solution. This process was followed by measuring the cyclic voltammogram.