A electrochemical surface forces apparatus (EC-SPA) was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) and oxidized ferrocene (ferrocenium cation, Fc(+)-SAM) in a 1 mM aqueous electrolyte. The double-layer repulsion in both cases of the Fc-SAM and Fc(+)-SAM electrodes was observed. The surface charge density (sigma) evaluated from the double-layer repulsions between the Fc(+)-SAM electrodes in 1 mM aqueous KClO4 was 0.0027 C/m(2), which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m(2). The sigma values of the Fc(+)-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m(2) for NO3-, SO42-, and CF3SO3-, respectively. The degrees of dissociation (alpha(d)) between the ferrocenium cation and these counteranions were obtained from sigma and the density of the ferrocenium on the electrode. The alpha(d) value of CF3SO3-, 4.1%, was the highest, followed in the order, SO42- > NO3- > ClO4-, indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.