The quantity of absorbed electrolyte and the electrical membrane conductivity have been measured and correlated for three cation-exchange membranes (CM2, CMx and MK-40), two electrolytes (KCl and LiCl), over a large concentration range of the solution (0.1 M less than or equal to C-0 less than or equal to 3.0 M). The membrane conductivity has been measured according to a French standard. However, because of the lack of standards fixing all the operating conditions, we established an experimental protocol, validated by a statistical study. This method will be proposed to be standardized for this category of measurements. Relationships have been established between the two parameters allowing us to determine both the counter-ion and the co-ion diffusion coefficients in a cation-exchange membrane, considered as homogeneous. The values of these coefficients for the counter-ions are in good agreement with those obtained from other methods. The variations of the computed diffusion coefficients of counter-ions and co-ions for the different studied systems are discussed in terms of internal interactions with the charged polymer matrix and the other diffusing species. The discussion also takes into account the free water content of the polymer.