N-Methyl-N-propylpiperidinium bis(trifluoromethanesulphonyl)imide (MePrPipNTf(2)), as well as its mixture with a molecular liquid acetonitrile (MeCN) were tested as electrolytes for carbon-double-layer capacitors. The conductivity of the MePrPipNTf2 neat ionic liquid is at the level of 1.5 mS cm(-1), while the corresponding value for the mixture (48 wt.% MePrPipNTf(2) + 52 wt.% of MeCN) is ca. 40 mS cm(-1) (both recorded at 25 degrees C. The electrochemical stability of the electrolyte (both neat ionic liquid, as well as its mixture with acetonitrile), detected at the glassy carbon electrode, was as broad as 5.7 V However, the tested capacitors with activated carbon as the active electrode material show considerable lower stability, reduced to ca. 3.7 V. The specific capacity estimated from both cyclic voltammetry and charging/discharging experiments was ca. 140 F g(-1), and after ca. 700 cycles, decreased to ca. 100 F g(-1). The effect of the loss of the part of initial specific capacity is probably due to the loss of Faradaic pseudo-capacity. The specific energy of the activated carbon in the tested devices was at the level of 240 kJ kg(-1) while the specific power density was ca. 25 kW kg(-1). (C) 2007 Elsevier B.V. All rights reserved.