A 2:1 smectite-type clay containing thiol groups on a basal plane was synthesized by reacting magnesium chloride with mercaptopropyltrimethoxysilane in an alkaline methanol/water mixture. The material (denoted as an HS-Mg layer) was identified by elemental analyses and X-ray diffraction measurements. The HS-Mg layer was revealed to be self-assembled on a gold surface. The process was confirmed by monitoring the increase in mass of a gold electrode with a quartz crystal microbalance when the electrode was immersed in a chloroform dispersion of the HS-Mg layer. Nanoscale images of a gold surface treated with the HS-Mg layer were obtained by atomic force microscopy. The 400 x 400-nm image of the sample showed that the surface was covered with platelets of about 30 mm in diameter and 1.9 nm in thickness. The 3.00 x 3.00-nm image showed that the bright spots on the surface of the particle displayed periodicity. The spots were ascribed to the alkanethiol groups on the silicate sheet of the HS-Mg layer, The observed periodicity coincided with the tetrahedral sheet of a model smectite clay.