The behaviour Of L-alpha-lecithin (DPPC) was studied at the water/1,2-dichloroethane interface within a range of interfacial potential differences below the zero-charge potential difference, using surface tension and voltammetric measurements. The general five-step model, established in Parts I and II of this series, has been confirmed. The surface pressure of DPPC adsorbed at the interface exhibits a maximum in acidic media close to the zero-charge potential difference, E-pzc, of the base electrolytes. It is demonstrated that, in contrast to the potentials higher than E-pzc, where the change in the surface pressure is due to desorption of the protonated form of DPPC, the observed change in the surface pressure at interfacial potential differences below E-pzc can be explained by alterations in the concentrations of the adsorbed zwitter-ionic and protonated forms of DPPC governed by the general acid/base equilibrium. (c) 2007 Elsevier B.V. All rights reserved.