The solid proton conductor zirconium phosphate sulfophenylenphosphonate of composition Zr(HPO4)(0.65)(SPP)(1.35) where SPP denotes metasulfophenylenphosphonate was prepared in the amorphous gel form in dimethyl formamide (DMF) and characterized by P-31 NMR. The composite membranes of SPEEK up to 50 wt.% of zirconium phosphate sulfophenylenphosphonate content were prepared by introducing the solid proton conductor from the gel. The composite membranes were characterized using Fr-IR, powder X-ray diffraction, SEM, DSC/TGA. The proton conductivity of the membranes was measured under 100% relative humidity up to 70 degrees C. The composite membranes had better thermal stability when compared with that of SPEEK. A three-fold increase in proton conductivity at 70 degrees C was observed for the composite membrane with 50 wt.% of solid proton conductor. Furthermore, the conductivity results imply that a critical percentage of proton conductor is needed to establish conduction pathways in the polymer matrix. (c) 2006 Elsevier B.V. All rights reserved.