Palladium composite membranes were synthesized using the supercritical impregnation method in order to reduce their methanol permeation properties in direct methanol fuel cells. Pd(II)(acetylacetonate)2 was dissolved in supercritical CO2 and impregnated into the Nafion membranes. The impregnated Nafion membranes were converted to the palladium deposited Nafion membranes by treating them with various concentrations of a reducing agent, sodium borohydride (NaBH4), at 50 degrees C for 2 h. The morphology of the surface and palladium distributions of the Pd/Nafion composite membranes were investigated by transmission electron microscope (TEM), electron dispersive spectrometry (EDS) and electron probe micro analysis (EPMA). To evaluate the palladium loading, the membranes were analyzed by ICP-AES. The prepared palladium composite membranes were characterized by measuring their methanol permeabilities and ion conductivities. Then, the characterized properties were compared with those of Nation, Finally, the performance of a cell employing the palladium composite membrane was evaluated using a DMFC single cell. (C) 2007 Elsevier B.V. All rights reserved.