Properties of a γ-butyrolactone-acetonitrile solution of triethylmethylammonium tetrafluoroborate were systematically measured for its potential to be used as an electrolyte in double-layer capacitors and for demonstrating the ways in which these properties can be optimized through choosing an appropriate solvent mixture and an appropriate salt and adjusting the electrolyte composition. These properties included the liquid range of the solvent and the solution at different compositions, the dielectric constant of the binary solvent as a function of solvent composition and temperature, the conductivity of the solution as a function of salt content, solvent composition, and temperature and the oxidative stability of the electrolytes at different solvent compositions. To correlate the properties of the electrolytes with the properties and performance of the double-layer capacitors utilizing the electrolytes, such capacitors were built and their equivalent series resistance, cyclic voltammogram, and operating voltage were measured for different solvent compositions of the capacitor electrolytes. The results revealed a clear correlation between the conductivity and the oxidative stability of the electrolytes and the equivalent series resistance and the operating voltage of the capacitors, respectively. Furthermore, it is clearly demonstrated that by using a binary solvent and adjusting its composition properly, the electrolyte it makes with an appropriate salt can be effectively optimized with regard to its liquid range, conductivity, and electrochemical stability window, which in turn imparts the double-layer capacitor utilizing such an electrolyte with a set of optimal properties such as a wide range of operating temperature, low equivalent series resistance, and high operating voltage.