The behaviour towards electron transfer of self-assembled monolayers of 2-mercaptobenzimidazole (MBI) and 2-mercaptobenzimidazole-5-sulfonate (MBIS) on Au(111) was examined by cyclic voltammetry. The influence of the monolayers was drastically dependent on the charge of the redox probe used. When [Ru(NH3)(6)](3+) is used, a post-adsorption peak characteristic of the adsorption of the redox probe is detected only at the MBIS modified electrode. Taking advantage of this difference, ac voltammetry has been used to deter-mine the surface composition when mixed monolayers are formed by immersion of the gold substrate in mixtures of different molar fractions of MBI and MBIS. Results clearly indicate that the ionic strength of the immersion solution plays a key role in the surface composition when a charged surfactant is mixed with non-charged surfactant. (c) 2006 Elsevier B.V. All rights reserved.