Proton conductors operating at intermediate temperatures are receiving significant attention due to their advantages over conventionally used materials in proton exchange membrane fuel cells. CsH2PO4 has proven to be proton conducting above 230 degrees C, however within a narrow temperature range of the superprotonic phase. In the present work CsH2PO4-ZrO2 composite electrolytes were prepared and investigated in the temperature range from 120 degrees C to 300 degrees C. As a result of improved thermal and mechanical stability the composite electrolyte exhibited high proton conductivities preserved to higher temperatures up to 280 degrees C under low atmospheric humidification. Higher open circuit voltage and stability in the extended temperature range were achieved with composite electrolytes with a CsH2PO4 to ZrO2 molar ratio of 2. (C) 2013 The Electrochemical Society.