A series of organic-inorganic hybrid membranes based on phosphoric acid and hyperbranched aliphatic polyester (Boltorn (TM) H2O) used as electrolytes for polymer electrolyte membrane fuel cells (PEMFC) operated at the temperature higher than 100degreesC have been prepared via a sol-gel process, and characterized using P-31 MAS NMR. Boltorn (TM) H2O was doped into the hybrid membranes at different levels to provide high hydrophilic and water conservation property. A high proton conductivity was obtained with suitable H2O doping level, and the membrane with the highest proton conductivity remained a conductivity of 5 x 10(-4) S cm(-1) even after holding for 7 h at 130degreesC under 30% relative humidity. The TGA/DTG results have shown that the obtained hybrid membranes were stable up to 200degreesC. (C) 2004 Elsevier B.V. All rights reserved.