A detailed study of the influence of electrolyte activity on apparent formal potentials measured for self-assembled monolayers of ferrocenylhexanethiol at high coverages on gold electrodes is provided. Ion selective electrodes are used in a logical sequence of comparisons to eliminate liquid junction potentials and, ultimately, to provide a rigorous comparison of apparent formal potentials for the monolayer over a broad range of concentrations in aqueous solutions of HClO4 and NaClO4. The results obtained in HClO4 and NaClO4 solutions are indistinguishable. Under the conditions examined it appears that protons and sodium ions have little influence on the apparent formal potentials. Hence it seems that protons and sodium ions have little influence on the overall interfacial potential distribution and, thus, little influence on the overall ionic structure near the monolayer/solution interface. The picture that emerges for the oxidized "monolayer" is one in which the high surface charge produced by the high surface coverage of ferroceniums is compensated by a highly compact layer of counterions.