The invention relates to a process for the extrusion of thermoplastic polymers having alkaline ionic groups.The process consists in preparing a mixture composed of a thermoplastic polymer having alkaline ionic groups and a plasticizer, in extruding the mixture obtained to form a film; then in washing the film obtained in aqueous medium to remove said plasticizer(s). The plasticizer is chosen from non-volatile compounds which are stable with respect to the ionic groups of the polymer, which are soluble in water or in solvents that are miscible with water, said plasticizers being chosen from the compounds that react with the ionic group of the polymer via formation of a weak bond of the hydrogen bond-type, and the compounds that react with the ionic group of the polymer via formation of a strong bond, of the ionic bond-type.