One of the factors that limits the lifetime of proton exchange membrane fuel cells (PEMFC) is the chemical stability of the polymeric membrane electrolyte operating at cell conditions. This work studies the thermal stability of a novel family of electrolyte membranes for use in fuel cells. The membranes were prepared by two different methods: Firstly, the MEB series were prepared by the addition of sodium dodecyl sulphate (SDS) to the resin obtained from the polymerization of terephthalic and adipic acid with glycerol. Secondly, the MP series were prepared from the polymerization of maleic acid and glycerol, phosphotungstic acid (PWA) was afterwards added to this material. The composites were processed in a reactor and shaped by hot pressing, yielding homogeneous and flexible slabs with an excellent surface finish. The materials were characterized by Thermo-gravimetric analysis (TGA), Scanning-electron microscopy (SEM), and electric conductivity measurements. The developed materials showed good properties as electrolytes for PEM fuel cells operating at temperatures above 100 °C.