We demonstrate how electrochemical impedance spectroscopy (EIS) measurement of the capacitance can be used to calculate the composition of mixed self assembled monolayers (SAMs) consisting of binary mixtures of octanethiol (OTT) with hexadecanethiol (HDT) and dodecanethiol (DDT) with 11-mercaptoundecanoic acid (MUA) on gold. We monitor as a function of immersion time changes in composition of the mixed SAM and examine the displacement of one thiol in a SAM with another thiol from solution. Under the experimental conditions the composition of the mixed SAM formed by co-adsorption from a mixture of thiols in solution is determined by thermodynamic, not kinetic, factors given sufficiently long immersion times (up to 10 days). Fully formed SAM consisting of a single component behave differently in that some pure SAM (e.g., DDT) are completely displaced when exposed to a competing thiol in solution, whereas others (e.g., MUA) can only be partially displaced, presumably because of strong interlayer cohesion between the carboxylate end groups. (C) 2005 Elsevier B.V. All rights reserved.