By modification of surfaces of multi-walled carbon nanotubes with ultra-thin monolayer-type films of phosphododecamolybdic acid, H3PMo12O40, an electrode material with improved capacitance properties is produced. It is apparent from three distinct test experiments (based on cyclic voltammetry, galavanostatic charging-discharging and AC impedance) that capacitors utilizing H3PMO12O40-modified carbon nanotubes are characterized by specific capacitances and energy densities on the levels of 40 Fg(-1) and 1.3 Wh kg(-1), whereas the respective values for the systems built from bare carbon nanotubes are lower, 22 Fg(-1) and 0.7 Wh kg(-1). It is reasonable to expect that fast and reversible multi-electron transfers of the Keggin-type H3PMo12O40 account for the pseudocapacitance effect and significantly contribute to the observed overall capacitance. (c) 2007 Elsevier Ltd. All rights reserved.