The use of nonhalogenated electrolyte salts for electrochemical devices is very important from the point of view of safety and cost. We have investigated the physical and electrochemical properties of I series of quaternary ammonium bis(oxalato)borates (QABOBs) and the performance of electric double-layer capacitors (EDLCs). The mass and van der Waals Volume of BOB- are hi.-her and larger than those of BF4-. The mass densities, viscosities, and Surface tensions of propylene carbonate (PC) Solutions containing the QABOBs are higher than those obtained for tetraethylammonium tetrafluoroborate (TEABF(4)), and the electric conductivities become lower. However, the electrochemical voltage windows measured with an activated carbon electrode and the gravimetric capacitances of 2025-type coin cells are comparable to those obtained for the PC solutions containing the TEABF(4). (c) 2006 Elsevier Ltd. All rights reserved.