Ionomeric membranes comprising (per)fluorinated, se-micrystalline or amorphous, ionomeric polymers, having equivalent weight (EW) from 380 g/eq to 1,800 g/eq, which used in fuel cells, under the following conditions: membrane thickness 50 μm assembled between two electrodes catalyzed with 0.6 mg/cm2 Pt/C and treated with 0.7 mg/cm2 of Nafion®, having a 10 cm2 area; hydrogen and air feeding, both at the pressure of 0.25 MPa, both saturated with water at 80° C.; cell temperature 75° C.; for membranes formed of copolymers TFE/F2C═CF—O—(CF2)2—SO2F, give the following maximum specific power (PMAX) values, at the indicated EW values: at EW=670 PMAX higher than 0.55 Watt/cm2; at EW=830 PMAX higher than 0.66 Watt/cm2; at EW=1,160 PMAX higher than 0.50 Watt/cm2; at EW=1,600 PMAX higher than 0.32 Watt/cm2.