Ordered 11-mercaptoundecanoic acid (MUA) structures formed from self-assembly on evaporated gold electrodes were probed with potassium ferricyanide through electrochemical titration. The ferricyanide response was limited at these highly blocking films at low salt concentrations necessitating addition of 1 M KCI electrolyte. Voltammetry results strongly support the penetration of ferricyanide probe molecules into the SAM films for electron transfer. Recent models for electrochemical titration of SAMs, and interactions in weak polyelectrolytes are evoked for data analysis. The pK(a) determined using the electrochemical titration model was 5.6 +/- 0.2 for 50% MUA/50% 6-mercaptohexanol monolayers and 5.8 +/- 0.2 for the 100% MUA films. Utilizing a statistical mechanical model for weak polyelectrolytes at the water-solid interface (1 - pK) and a pK of 5.7, an intermolecular interaction component (beta (E) over bar) of 2 was obtained for the 100% COOH films and beta (E) over bar of 1 for the 50% COOH films, which suggests a more deprotonated 50% MUA film with less intermolecular interactions compared to pure MUA films. 2012 Elsevier B.V. All rights reserved.