Properties of the homogeneous cation exchange membranes CMS and CMV were investigated in order to assess the applicability of the model of micro-heterogeneous structure in the case of these membranes. Electrical conductivity measurements provide the volume fractions of the two distinct internal phases that are supposed to compose the entire membrane volume. These partial volumes as well as the membrane overall swelling depend on the nature of the external solutions. In order to extend our knowledge concerning the role of each of these phases, the absorption isotherms of H2SO4 and NaCl have been measured in relation to the membrane water content. We have determined experimentally a partition coefficient at absorption equilibrium between the external solution and the so-called interstitial phase. With the aim of clarifying the condition of validity of the Donnan theory in terms of the micro-heterogeneous model, we have developed a treatment involving a more detailed description of the water and salt distribution at equilibrium between the interstitial phase and the gel phase. This study accounts for the influence of different possible structural configurations of the poly-electrolyte macro-ions which are confined to the gel phase. The absorption properties of both membranes were also studied at equilibrium with a mixed H2SO4 + NiSO4 solution. Application of conductivity measurements to these systems has led to determine the H+ ion and Ni2+ ion mobility in both membranes and to bring out the comparison of these values in terms of the membrane internal structure. (C) 2008 Elsevier B.V. All rights reserved.