Inorganic-organic composite electrolyte membranes were fabricated from Cs_XH_3−XPMo₁₂O₄₀ (CsPOMo) and quaternary diazabicyclo-octane polysulfone (QDPSU) using a polytetrafluoroethylene (PTFE) porous matrix for the application of intermediate temperature fuel cells. The CsPOMo/QDPSU/PTFE composite membrane was made proton conducting by using a relatively low phosphoric acid loading, which benefits the stability of the membrane conductivity and the mechanical strength. The casting method was used in order to build a thin and robust composite membrane. The resulting composite membrane films were characterised in terms of the elemental composition, membrane structure and morphology by EDX, FTIR and SEM. The proton conductivity of the membrane was 0.04 S cm⁻¹ with a H₃PO₄ loading level of 1.8 PRU (amount of H₃PO₄ per repeat unit of polymer QDPSU). The fuel cell performance with the membrane gave a peak power density of 240 mW cm⁻² at 150 °C and atmospheric pressure.