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Recently a new method was introduced to operate an immunological field effect transistor (ImmunoFET). By changing the electrolyte concentration of the sample solution stepwise (the so-called ion-step), a transient diffusion of ions through the membrane-protein layer occurs, resulting in a transient membrane potential, which is measured by the ImmunoFET. It became apparent that the maximum of the membrane potential is a function of pH, owing to a pH-dependent charge density caused by the amphoteric nature of the embedded proteins in the membrane of the ImmunoFET. At a certain pH value, which is called the inversion point (pI'), the membrane potential changes sign. This inversion point is characteristic of the type of protein and the type of membrane and depends on the isoelectric point, the titration curve and the concentration of all amphoteric groups in the membrane. In this paper an attempt is made to establish a theoretical basis for the ion-step method. Because there i no model which describes ion transport in charged membranes and its dynamic behaviour as a result of the ion-step, an existing equilibrium theory has been adapted. The well-known Teorell-Meyer-Sievers (TMS) theory, which describes the membrane potential for charged membranes, is used as a framework. The adapted TMS model was verified by experimental data.