This work describes the design and testing of organic electrolyte systems that extend the low temperature operational limit of double-layer capacitors (also known as supercapacitors) beyond that of typical commercially available components. Electrolytes were based on a tetraethylammonium tetrafluoroborate/acetonitrile system, modified with low melting co-solvents (such as formates, esters and cyclic ethers) to enable charging and discharging of test cells to as low as -75 degrees C. Cell capacitance exhibited little dependence on the electrolyte salt concentration or the nature of the co-solvent used, however, both variables strongly influenced the cell equivalent series resistance (ESR). Minimizing the increase in ESR posed the greatest design challenge, which limited realistic operation of these test cells to -55 degrees C (still improved relative to the typical rated limit of -40 degrees C for commercially available non-aqueous cells). (C) 2007 Elsevier B.V. All rights reserved.