Hybrid polymer electrolytes based on copolymers of vinylidene fluoride, (VdF) and different molar ratios of hexafluoropropylene (HFP), and an electrolyte solution ethylene carbonate (EC)/propylene carbonate (PC)/ LiN(CF3SO2)(2) have been prepared by the solvent casting technique. Thermal analysis showed that small or no interactions exist between the electrolyte solution and the host polymer matrices. Conductivities in excess of 10(-3) S/cm at room temperature were obtained. However, on isothermal heating at 55 degrees C under vacuum, the samples showed relevant losses of the electrolyte solution, that caused the conductivity to drop by 1-2 orders of magnitude. Cationic transport numbers, t(+), ranging between 0.2 and 0.3 have been measured by the Hittorf method. (C) 1998 Elsevier Science Ltd. All rights reserved.