The effect of the applied potential across a biomimetic model membrane, Deltaphi, on the extent of dissociation of surface bound ionizable groups has been examined theoretically using a formalism that shares common features with that recently introduced by White and coworkers (Langmuir 9 (1993) 1) to account for the occurrence of peaks in the cyclic voltammetry of self-assembled monolayers rigidly attached to an electrode surface, bearing the same type of ionizable groups facing the electrolyte. Numerical solutions of the governing coupled, highly non-linear system of equations yielded for reasonable membrane parameters, and under conditions of physiological relevance, linear changes in the extent of ionization as a function of Deltaphi, about Deltaphi = 0, of ca. 0.07 units/V. Also considered in this work was a more general situation in which the contributions to the total interfacial capacity, C-T, due to the diffuse double layers, C-S(j), and to the fixed charges C-j(f(j)) on each of the sides of the membrane-like assembly denoted by 'j', were comparable in magnitude to the potential independent capacity of the intervening hydrophobic layer, i.e. low-electrolyte concentration. In such case, plots of C-T versus Deltaphi displayed a global minimum at Deltaphi = 0 and two local maxima at values of Deltaphi slightly negative and positive to the maxima observed in the corresponding C-j(f(j)) versus Deltaphi plots. Implications of the results obtained to excitable bilayer membranes are briefly discussed. (C) 2003 Elsevier Ltd. All rights reserved.