Mg-Al layered double hydroxides (LDHs) intercalated with carbonate anions were prepared by the co-precipitation process with Mg/Al = 2, 3 and 4, and the effect of Mg/Al ratio on the ionic conductivity was examined. It is shown that the Mg/Al ratio influences ionic conductivities of Mg-Al CO32- LDH. The conductivity of Mg-Al CO32- LDH with Mg/Al = 2 and 3 exhibited almost the same ionic conductivity of about 1 x 10(-3) S cm(-1) at room temperature under 80% relative humidity, while that of LDH with Mg/Al = 4 was lower by one order of magnitude. Thermal analyses showed that Mg-Al CO32- LDH with Mg/Al = 2 or 3 keeps interlayer water molecules at higher temperatures compared with Mg/Al = 4, suggesting that the interlayer water molecules are strongly held in the interlayer. The strongly held interlayer water molecules are assumed to contribute the high ionic conductivity in Mg-Al CO32- LDH with Mg/Al = 2 and 3. The alkaline type direct ethanol fuel cells using Mg/Al CO32- LDHs as hydroxide conducting electrolyte were confirmed to operate, and larger power was obtained in Mg-Al CO32- LDH with smaller Mg/Al ratio. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.007204jes] All rights reserved.