An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF4 −) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF3SO2)2N−] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF4) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF4 is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0V) and high ionic conductivity (4.8mScm−1 at 25°C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF4 as the electrolyte. This EDLC (working voltage ∼2.5V) has both, a higher capacity above room temperature and a better charge–discharge cycle durability at 100°C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.