A 2:1 smectite-type clay containing thiol groups on a basal plane was synthesized by reacting nickel chloride with (3-mercaptopropyl)trimethoxysilane in an alkaline methanol/water solvent. The material (denoted as an HS-Ni layer) was revealed to be self-assembled on a gold surface. The process was confirmed by the mass increase of a gold electrode with a quartz crystal microbalance (QCM) when the electrode was immersed in a chloroform suspension of an HS-Ni layer. The nanoscale images of a gold surface modified with an HS-Ni layer were obtained with an atomic force microscope (AFM). The surface was covered with a single layer of the platelets of about 30 nm in diameter and 1.6 nm in thickness. When the cyclic voltammogram was measured on an electrode modified with such a layer, the reversible redox peaks were observed in the potential range 0-300 mV (vs SCE). The peaks were ascribed to the reduction-oxidation of nickel(II)/nickel(I) couple. From the Coulometric measurements, the fraction of the electroactive nickel center was estimated to be 9.5% of the whole Ni(II) ions.