Process for preparing ionomeric membranes

A process to prepare ionomeric membranes formed by (per) fluorinated ionomeric electrolyte polymers salified with a lithium ion, comprising the following steps: a) obtaining of (per) fluorinated polymer membranes, containing —SO2F groups pendent with respect to the main chain; b) partial or complete salification of the (per) fluorinated polymer membranes containing —SO2F groups with basic lithium compounds and final water washing; and c) swelling and contemporaneous drying of the membranes by dipping in a heterogeneous biphasic system consisting of: (1) a dipolar aprotic organic solvent or a mixture of dipolar aprotic organic solvents, and (2) molecular sieves dispersed therein which are inert with respect to the solvent and the membranes; and wherein any swelling step in the process occurs in the absence of protic solvents.