A self-assembled monolayer (SAM) of naphthalene disulfide (NDS) has been formed on a polycrystalline gold surface. Techniques like cyclic voltammetry and impedance measurement with an external redox probe along with scanning tunnelling microscopy (STM) are used to investigate the monolayer structure and ion permeation through it. Impedance analysis shows that the rare constant or a fast redox couple like Fe(CN)(6)(4-/3-) has decreased from 2.3 x 10(-2) to 9.4 x 10(-4) cm s(-1) due to the barrier provided by the monolayer. The STM image clearly shows the presence of defects (as pinholes) on the monolayer surface in agreement with the residual current observed in the voltammograms.