An alkyl disulfide containing two sulfonate-substituted ferrocenes, 7,8-dithiatetradecane-1,14-di(aminocarbonyl)-bis(1＇-ferrocene-l-sulfonic acid), was synthesized and used to form electroactive self-assembled monolayers on gold electrodes. Atomic force microscope (AFM) force curves were employed to measure in situ the change in surface charge as the ferrocene groups were oxidized to compensate for the negative charges on the sulfonate groups. The electrode surface charge density was calculated from the surface coverage measured by electrochemical oxidation of the ferrocene groups, while the diffuse double layer charges were obtained from theoretical fits of the force data to solutions of the complete nonlinear Poisson-Boltzmann (PB) equation with knowledge of the silica probe surface potential. A significant difference between the AFM measured (i.e., effective) surface charge and the surface charge calculated from electrochemical measurements was found. This difference is attributed to a layer of counterions near the surface that screens a large fraction of the surface charge (variously described as "ion condensation" or failure of the nonlinear PB theory). The experimental results also showed that the extent of this ion screening (similar to 97%) was relatively constant and independent of the total electrode surface charge.