n-Alkanethiols (CH₃(CH₂)_nSH, n=3, 5, 11, 13, 15, 17) have been adsorbed at mercury electrodes prom ethanolic solutions. A. c. voltammetry has been carried out of the adsorbed alkanethiol on mercury electrodes in aqueous solutions containing 1M KCl at 25°C. Cyclic d.c. voltammetry of several metal ions also has been performed at these electrodes. The results show that long-chain alka-nethiols n_??_11 can form densely packed monolayers with fully extended alkyl chains in the potential range between 0 and -0.7 volt vs. Ag/AgCl. Metal ions such as Pb²⁺ are unable to penetrate into the monolayer. In contrast, short-chain alkanethiols with n=3 and 5 form less ordered and loosely packed adsorption layers, which are permeable to the metal ions. A pH dependent a.c. voltammetric peak is observed at a mercury electrode with adsorbed alkanethiol(n=5) in buffer solutions, which is attributable to the redox reaction of the adsorbed thiol: [CH₃(CH₂)₅S]_adHg+H⁺+e^-_??_[CH₃(CH₂)SH] _adHg.