The steady-state electrical behavior of cation-selective ionophore membranes with different kinds of ionic additives was studied on a theoretical basis. Membranes with both mobile and fixed anionic sites exhibit nonlinear current-voltage curves, which is in contrast with the ohmic behavior of pure fixed-site membranes. This can be explained from the specific concentration profiles in the membrane, illustrating pronounced polarization effects for the mobile sites. Surprisingly, the influence of the fixed sites on the current response is still observable even when the mobile sites predominate. At high voltages, current saturation occurs by limitations of the back-diffusion of free ionophores. This can be overcome when other, uncomplexed, cations contribute to the current flow. Membranes with both cationic and anionic sites cannot be strongly polarized, in analogy to other systems where an inert electrolyte is confined to a given phase. These membranes respond with current saturation at relatively low voltages. Currents above this limit can be achieved by an additional permeation of anions from the aqueous solution through the membrane, as shown by model calculations. © 2005 Elsevier B.V. All rights reserved.