Reactions of electroactive species adsorbed on electrodes from surfactant solutions are influenced by coadsorbed nonelectroactive surfactant. Theory and methods are described to provide Frumkin interaction parameters between all amphiphilic species adsorbed onto the electrode in such situations, using cyclic and square wave voltammetry (SWV) and assuming reversible electron transfer. A simple equation to estimate surface concentrations from reversible SWV curves is presented. The method was applied to the electroactive amphiphile dimethyl(ferrocenylmethyl)hexadecylammonium ion (Fc-C16) adsorbed onto glassy carbon electrodes from aqueous solutions containing hexadecyltrimethylammonium bromide (CTAB). Results were consistent with head-down orientations of these surfactants on a glassy carbon electrode. While mixed adsorbate layers were formed, a small preference for adsorption of Fc-C16 was found in the presence of CTAB. Interaction parameters between Fc-C16/Fc-C16, Fc-C16/CTAB, reduced and oxidized forms of Fc-C16, and CTAB/CTAB were attractive, suggesting favorable hydrophobic interactions. Interactions of the +2 charged, oxidized Fc-C16 with itself and with CTAB were repulsive, suggesting that Coulombic head group repulsion overcomes hydrophobic interactions between these species.