The electrical properties of single-walled carbon nanotubes (SWNTs)/TlH2PO4 composites have been studied, a metallic transition being observed around a SWNTs concentration of 0.2 wt.% and investigated by means of impedance spectroscopy. This work represents the first study of its kind to show that nanoparticles may be employed as valued additives that are compatible with the solid acids, providing for increased electrical conductivity of fuel-cell electrolytes. A two orders of magnitude increase in the surface capacitance was observed in the composite, opening up a possibility of new solid acid electrolytes for fuel cells and composite electrolytes serving as a supercapacitor by itself. (C) 2010 Elsevier B.V. All rights reserved.