Ionic liquids like 1-butyl-3-methylimidazolium tetrafluoroborate (IMIBF4) or hexafluorophosphate (IMlPF(6)) and 1-butyl-4-methylpyridinium tetrafluoroborate (PyBF4) were mixed with organic solvents such as butyrolactone (BL) and acetonitrile (ACN). A lithium salt (LiBF4 or LiPF6) was added to these mixtures for possible application in the field of energy storage (batteries or supercapacitors). Viscosities, conductivities and electrochemical windows at a Pt electrode of these electrolytes were investigated. All studied electrolytes are stable toward oxidation and exhibit a vitreous phase transition, which has been determined by application of the VTF theory to conductivity measurements. Mixtures containing the BF4- anion exhibit the lowest viscosity and the highest conductivity. Two mixtures have been optimized in terms of conductivity: BL/lMIBF(4) (60/40, v/v) and ACN/IMIBF4 (70/30, v/v). (c) 2005 Elsevier B.V. All rights reserved.