Composites made from RuO2 center dot xH(2)O particles supported on carbon nanofibres (CNF) have been prepared for supercapacitor electrodes. CNF, produced by Grupo Antolin Ing. SA. using a floating catalyst procedure was treated either in HCl or in HNO3. Then the composites were obtained by impregnation of CNF with an aqueous RuCl3 center dot 0.5H(2)O solution followed by filtering and alkali solution treatment. Heat treatment at 150 degrees C for 2h was done. Specific capacitance of the composites has been measured and discussed on the basis of their RuO2 center dot xH(2)O content and RuO2 center dot xH(2)O particle size. The composites having RuO(2 center dot)xH(2)O contents below 11 wt% show RuO2 center dot xH(2)O particles, which grow from 2 to 4 nm as the RuO2 center dot xH(2)O content increases. The specific capacitance of supported RuO2 center dot xH(2)O, which can be very high (up to 840 Fg(-1)), decreases as the RuO2 center dot xH(2)O content increases and RuO2 center dot xH(2)O particles grow. The composites having RuO2 center dot xH(2)O contents above 11 wt% show RuO2 center dot xH(2)O particles of nearly constant size (4 nm); the effect of increasing the RuO2 center dot xH(2)O content is to increase the amount of particles but not the size of the particles. In these composites the specific capacitance of supported RuO2 center dot xH(2)O is nearly constant (440 F g(-1)) and close to bare RuO2 center dot xH(2)O (460 F g(-1)). (C) 2007 Elsevier B.V. All rights reserved.