The structure and electrochemical properties of pure and mixed monolayers of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) self-assembled on a gold disk electrode (AuE) have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes containing a one-electron redox couple. These results are compared with those obtained by cyclic voltammetry (CV). The monolayer surface coverage was determined using impedance measurements through the charge-transfer resistance, and by the pore size model. Cyclic voltammetry considering gold oxide formation/removal, as well as the oxidation/reduction process of a redox probe, was also employed to calculate the surface coverage. Taking into account the stability of the studied monolayers during the electrochemical experiment, the comparison of the barrier to charge transfer at both, the modified and the unmodified electrodes, resulted to be an appropriate method for estimating the surface coverage percentage given by each of the considered monolayers. The obtained results demonstrated that the MPA-monolayer-coated electrode behaves like a microelectrode array, with pinholes acting as the microelectrodes. The average size and the separation of the pinholes, estimated by the pore size model, are 1.9 and 21 mu m, respectively. However, MUA forms a closely packed monolayer that acts as an ionic insulator and is virtually defect-free. The adsorption kinetics of MPA and MUA SAMs on the gold electrode (AuE) was evaluated. As expected, mixed monolayers of MPA and MUA show characteristics of both, long and short-chain alkanethiol monolayers. (c) 2005 Elsevier B.V. All rights reserved.