Thermodynamics for the binding of electrolyte cations, in particular, tetraalkylammonium ions, to the electrogenerated radical anion and the dianion of the fullerene moieties in a molecular-bilayer membrane film of C-60-bearing artificial lipid, 1, cast on an electrode surface was described. The theoretical treatment predicts a linear relationship between the half-wave potential (E-1/2) for the electrode reaction and electrolyte concentration in the film (c) at higher concentrations and enables us to obtain the number of bound cation and binding constant, respectively, from the slope and intercept of the E-1/2-ln c plot. We measured differential pulse voltammograms for a cast film of 1 on a basal plane graphite electrode in aqueous solution containing tetraethylammonium chloride (or tetra-n-butylammonium chloride) + KCl = 0.5 M and found that E-1/2-ln c, plots for the modified electrodes are linear in a concentration range of ca. 0.002-0.14 M, where c(s) is the concentration of ammonium electrolytes in the solution phase. The analysis of the plot reveals that the fullerene radical anion forms a 1:1 complex with both tetraethylammonium and tetra-n-butylammonium cations and that the fullerene dianion forms a 1:2 complex with tetra-n-butylammonium cations. The binding constants K-1 for the binding of tetraethylammonium and tetra-n-butylammonium cations to radical anions of the fullerene moieties was determined to be 7.93 x 10(3) and 1.71 x 10(8) M-1, respectively, provided that the partition coefficients (c/c(s),) of the tetraalkylammonium ions are unity.