Alkanethiol monolayers were formed on gold at different potentials by a potential-controlled assembly procedure, which involves (1) reductive desorption of the pre-adsorbed thiol molecules to "clean" the gold surface, and (2) subsequent electrochemically directed adsorption of thiol molecules at positive potentials. The dependence of the monolayer integrity on the substrate potential was investigated using grazing incidence reflection-absorption Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is demonstrated that the substrate potential exerts a great effect on the quality of alkanethiol monolayers. The monolayers formed at approximate to 0.4 V (us SCE) have the best film integrity. A mechanism is proposed to explain the effect of substrate potential on the film quality. This work provides solid basis for the selection of adsorption potentials in potential-controlled formation of organic layers on solid surfaces. (c) 2006 Elsevier B.V. All rights reserved.