The invention relates to energy storage devices such as capacitors and supercapacitors and non-aqueous solvent systems suitable for use as an electrolyte solvent therein. Devices incorporating the solvent system are suitable for use in, for example, wireless devices or automotive applications at high temperatures with minimal, if any mass loss. The solvent system has at least one low boiling component (preferably a nitrile, eg acetonitrile) at least one high boiling component compatible with said low boiling component (preferably lactones, eg .gamma.-butyrolactone and/or organic carbonates eg ethylene carbonate or propylene carbonate); and wherein the components are selected in an amount such that said non-aqueous solvent system does not boil at the boiling point of the low viscosity solvent alone but has a boiling point greater than said low viscosity solvent alone.