This paper discusses the effect of adsorption of hydrogen and hydroxide ions which are present in the solution on the electrical capacitance of the bilayer membrane formed of phosphatidylcholine. The capacitance of the membrane was determined as a function of pH from electrochemical impedance spectroscopy measurements. A simple model describing the surface of the membrane build from phosphatidylcholine is presented. Model supposes the existence of four different forms of phosphatidylcholine as a result of adsorption of H+ and OH- ions on the bilayer surface. The contributions of the individual lipid molecule forms to capacitance of the membrane were assumed to be additive. Theoretical equations were derived to describe the electrical capacitance as a function of pH for assumed model. (c) 2012 Elsevier Ltd. All rights reserved.