Membrane potentials of weak acid ion exchange membranes separating almost neutral (pH = 5.7) solutions of NaCl, KCl and LiCl are studied both experimentally and theoretically. The space-charge model is applied in the theoretical description taking into account the proton flux, the variation of the effective fixed charge concentration in the transport direction and concentration polarization at the membrane/solution interfaces. For the case of perfectly stirred solutions, the validity of a general equation for the membrane potential similar to a well known relationship of non-equilibrium thermodynamics is discussed. Using the above model, the experimental results of membrane potentials of weak acid membranes are analyzed. These membranes were prepared by treating a cellulose acetate filter with a solution of a polyacid. Unstirred boundary layer effects are traced by comparing two types of potential measurements. Assuming free solution values of the diffusion coefficients and using a known value of the acid dissociation constant, respectively, the pore radius, the concentration of ionizable groups and a parameter related to the thickness of the unstirred layers are obtained. For the case of KCI solutions also, specific adsorption of K+ to the fixed negative charges (counterion binding) is considered. Finally, the influence of the electrolyte concentration on the conformational state of the adsorbed polyacid is also discussed from the results of hydrodynamical permeability measurements.