Because of recent discovery that the effect of onium cation symmetricity on its properties could be manipulated to benefit the application of an electrochemical double-layer capacitor, ii series of new unsymmetrical onium salts was synthesized and evaluated as electrolyte solutes in nonaqueous solvents. A relationship was established between the cation/anion structure and melting point as well as solubility of these salts. In order to obtain a detailed view about the effect of onium cation structure on its stability in capacitor application, we investigated the electrochemical stability window of the nonaqueous electrolytes based on these salts, using both a conventional stability window determination and a new approach especially defined for capacitor-like applications. (C) 2001 The Electrochemical Society. All rights reserved.