A process for preparing ionomeric membranes

A process for preparing membranes formed by (per) fluorinated ionomeric electrolytes salified with the lithium ion, comprising the following steps: a) obtaining of (per) fluorinated polymer membranes, containing --SO.sub.2F groups; b) partial or complete salification of (per)fluorinated polymer membranes containing --SO.sub.2F groups with basic lithium compounds and final washing with water; c) swelling and contemporaneous drying process of membranes by dipping in a heterogeneous biphasic system of a dipolar aprotic solvent wherein insoluble solid drying agents are dispersed.