In this work, we have measured the swelling properties (thickness, area, density, solvent content), and the conductivity of two different cation-exchange membranes (Nafion®117 and CM2), as functions of methanol content (XMeOH) and LiCl concentration in the external solution. The volume fraction of electroneutral solution in the membrane (f2) was found by using the microheterogeneous model of Gnusin. The values of f2 as well as the porosity of Nafion®117 obtained as functions of the methanol content are compared with those calculated on the basis of the nanostructure model developed by Haubold et al. Generally a good agreement is noticed. It is found that the volume of regions occupied by side chains with fixed ion-exchange groups increases with methanol content, that determines the total swelling and growth in porosity of Nafion®117. This swelling is accompanied however by a small decrease in the pore core region volume, free of polymer chains. The latter results in a decrease in f2, found also from conductivity measurements. The lower ionisation of the inner solution caused by the presence of less polar and less ionising solvent, such as methanol, produces a decrease in the membrane conductivity, which is partially compensated by swelling of side-chain regions. The conductivity (κ*) of CM2 membrane, which is reticulated and swells only slightly in the presence of methanol, decreases much more in comparison with non-reticulated and strongly swelled Nafion in pure methanol solution: