This study describes new electrolyte systems that utilize alkylated cyclic carbonates, with a primary focus on getting a higher withstand voltage for electric double-layer capacitors (EDLCs). We attempted to increase the oxidative durability of carbonate solvents by protecting the 4th and/or 5th positions of the five-membered carbonate ring; protection was achieved by substituting those positions with small alkyl group(s). We investigates six different types of cyclic carbonates, viz., ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), 2,3-butylene carbonate (2,3BC), isobutylene carbonate (iBC), and pentylene carbonate (P1C) have been investigated with regard to the electrochemical stability, as well as the melting point, boiling point, dielectric constant, viscosity, and the solubility of electrolyte salts. As a result, 2,3BC remained as the best potential candidate for an alternative solvent for EDLCs. 2,3BC has a high boiling point (243 degrees C) that is comparable to PC (242 degrees C) and dissolved spirobipyrrolidinium tetrafluoroborate (SBP-BF4). A SBP-BF4/2,3BC system showed a stabilized capacitance within wider voltage windows (Delta V = 3.5 V) that far exceeded that of conventional PC based systems (Delta V = 2.7 V). This high withstand voltage is caused mainly by the outstanding oxidative durability of 2,3BC. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.038112jes] All rights reserved.