Self-assembled monolayers of octadecanethiol (ODT) on gold have been studied by electrochemical impedance spectroscopy (EIS). The fractional coverage, pinhole size and separation have been examined as a function of immersion time of Au in the ODT deposition solution. The fractional coverage of ODT monolayer increases sharply from zero to more than 99% of its maximum within the first minute. However, it takes hours or a day for the fractional coverage to approach its final value. The pinhole separation increases from ca. 1.8 mum to 53 mum as the assembling time increases from 5 s to 24 h, indicating that the number of pinholes decreases. The pinhole radius increases slowly from ca. 0.3 mum to 1.6 mum in the same time interval. The slight trend of increasing pinhole size with adsorption time is attributed to the increasing influence of collapsed sites in the ODT monolayer.