The influence of small addition of acetonitrile (AN) (from 0.016 to 16 wt%) to 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)) electrolyte for electrical double layer capacitors (EDLCs), completed from two carbon cloth electrodes, has been studied using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and by modeling the EIS data with equivalent circuit. The addition of AN to the EMImBE(4) has strong influence on the bulk viscosity, conductivity and melting temperature of EMImBF(4), when more than 1.6 wt% of AN is added. Within the electrochemical stability region of EDLCs, the performance (gravimetric capacitance similar to 125 Fg(-1), characteristic relaxation time similar to 14 s, specific energy similar to 49 Wh kg(-1) or 12 Wh dm(-3), and specific power 28 kW kg(-1) or 7 kW dm(-3)) of the EDLCs based on EMImBF(4) and AN electrolyte is not improved when the addition of AN is from 0.033 to 3.3 wt%. However, the relaxation time is decreased to 8 s and specific power of EDLC is increased to 49 kW kg(-1), if 16 wt% of AN is added. More pronounced distortion effects and deviations from ideal capacitive behavior have been observed at higher applied potentials (3.2 V) when the addition of AN in the electrolyte of EDLC is higher than 3.3 wt%. The electrochemical stability of EDLC decreases through increase in the extent of faradaic reactions occurring at relatively high applied potentials, when larger amount of AN is added to the electrolyte EDLC. (C) 2012 Elsevier Ltd. All rights reserved.