Electrolyte for use in an energy storage device such as a capacitor or supercapacitor which comprises a solvent (preferably propionitrile) and an ionic species (preferably methyltriethylammonium tetrafluoroborate). The electrolytes provide a low ESR rise rate, a high voltage and permit operation over a wide range of temperatures, which makes them beneficial for use in a range of energy storage devices such as digital wireless devices, wireless LAN devices, mobile telephones, computers, electrical or hybrid electrical vehicles.