The KOH-ZrO2 solid electrolytes were prepared by sol-gel process. Ion conductivity and the conducting ion species were investigated from the impedance measurement and the electromotive force measurement. Resultant KOH-ZrO2 solid electrolyte showed high conductivity of 9 x 10(-3) S cm(-1) at 20 degrees C and 60% relative humidity. Hydroxide ion was considered as the main charge carrier from the potentials of the water vapor concentration cells. All-solid state iron/air secondary batteries were successfully prepared with the composite negative electrode composed of sponge iron, KOH-ZrO2 electrolyte and carbon black. (C) 2013 Elsevier B.V. All rights reserved.