Abstract The total transmembrane potential includes two surface potentials and an inner-membrane component. The separate calculation and measurement of these components has fundamental significance because they play different functional roles in trasport and enzyme processes. Surface potentials are defined by the well known double layer theory. Inner-membrane fields depend on the symmetry of the membrane and electrolyte solutions, their concentration and conditions in the electric circuit. In this paper the results of inner-field calculations for a non-symmetric electrolyte system and an open electric circuit are presented. Also the method is discussed which allows the measurement of the inner-field, the surface charge and the adsorption. This method, based on the dependence of the membrane capacitance on the electric field is used in two variations. In the first one a linear potential sweep is applied to the membrane, in the second one a sinusoidal. As a result, the surface charge of the membranes made from different lipids and their mechanical characteristics are obtained. The electric breakdown of the membrane is also discussed.