Electrolytes of LiBF4, LiPF6, LiBOB, Et4NBF4, and Et4NPF6 in propylene carbonate-diethyl carbonate (PC-DEC) and propylene carbonate-ethylene carbonate (PC-EC) binary solvents were compared for their electrolytic conductivity (kappa) and viscosity (eta) to assist their application to the batteries and capacitors and to understand the mechanisms underlying the differences in these properties. For eta, the Li+-salts resulted in considerably higher values than the Et4N+-salts, and the influence of an anion was such that the larger its size, the higher the eta of the electrolyte. Correspondingly, kappa of the Et4N+-electrolytes was substantially higher than the Li+-electrolytes, and a larger anion was accompanied by a lower kappa, except for electrolytes at the low-eta end. Toward this end, the order of kappa tended to be in reverse: the k of an electrolyte with a larger anion rose relative to one with a smaller anion. All evidence presented leads to the conclusion that the h, and thereby the k, of the electrolytes are dominantly affected by the number of unassociated Li+ ions through their solvation by the carbonate solvent molecules, the different anions of the Li-salts exerting their influence only through altering the number by their different powers for associating with the Li+. (C) 2004 The Electrochemical Society.