MgAl layered double hydroxide treated in a (CH3COO)(2)Mg aqueous solution (LDH-Ac) and composite membranes consisting of sulfonated poly(etheretherketone) (SPEEK) and the LDH-Ac in various volume ratios of 0-50 vol% were prepared. Their proton conductivities were examined at 40-150C degrees under 100 or 35% relative humidity (RH). The composite membrane with 12 vol % LDH-Ac showed a relatively high conductivity of 8.0 X 10(-3) S cm(-1) at 150 degrees C under 100% RH and a better dry resistance than the SPEEK membrane. The SPEEK/LDH-Ac composites were promising membranes for use above 100 degrees C. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3394435] All rights reserved.