The suitability of small additions (from 0.005 M to 1 M) of different organic solvents (acetonitrile, propylenecarbonate and gamma-butyrolactone) to improve the properties of 1-ethyl 3-methylimidazolium tetrafluoroborate (EMImBF(4)) as an electrolyte in electrical double layer capacitors (EDLCs) has been studied. The cyclic voltammetry and the electrochemical impedance spectroscopy (EIS) methods were used to determine electrochemical properties of EDLCs filled with the mixtures studied. The series capacitance, characteristic relaxation time, specific energy and power were calculated for the EDLCs tested and compared to the properties of EDLCs filled with pure EMImBF(4). To gain better understanding of the effect of organic solvent additive on the electrochemical properties of EMImBF(4) as an electrolyte in EDLCs an equivalent circuit fitting method was used to model the electrochemical impedance spectroscopy data. Although, the increased conductivity, decreased viscosity and melting temperature were observed for EMImBF(4) with a larger amount of the organic solvent additives, an unexpectedly negative effect of the organic solvent additives on the characteristic electrochemical properties established for EDLCs, which is demonstrated and explained through a full analysis of the electrochemical characteristics mentioned above. (C) 2013 The Electrochemical Society. All rights reserved.