NH4PO3–PTFE inorganic–organic composite electrolytes were prepared. Their phase compositions, thermal stability and morphologies were investigated. The composites obtained demonstrate improved thermal stability comparing with commercial NH4PO3. The morphology of pellets under the same conditions shows the smoother and denser surface with more PTFE content added. The conductivity was measured by ac impedance spectroscopy under different atmospheres. The composites were found to exhibit a higher conductivity in humid 5% H2/Ar compared with open air atmosphere. A maximum conductivity of 2.3 mS/cm is obtained at 250 °C for NH4PO3–PTFE (volume ratio = 2:1) sample in humid 5% H2/Ar.