The membrane potentials across cation- and anion-exchange membranes were measured for five LiBr-various-solvent systems. Water, dimethyl sulfoxide, ethylene glycol, methanol, and n-propanol were used as the solvents in this work. The experimental data were analyzed on the basis of the Donnan equilibrium and the Nernst-Planck equation of ion flux considering the effect of ion pairing of the electrolyte in the external solution. The Fuoss formalism of ion pairing was adopted to estimate the effective ion concentration in the external solutions, and the theoretical predictions agreed well with the experimental data. The effective membrane charge densities and the cation-to-anion mobility ratios in the membrane were also determined in this work. The results showed that the effective membrane charge density will vary with the kind of solvent. We attempted to introduce the concept of ion pairing inside the membrane to clarify this phenomenon.