As an electrolyte solvent for lithium ion capacitors (LICs), tetrahydrofuran (THF) showed lots of advantages on increasing the discharge capacity, suppressing the potential drop at charged state, and improving the slope of discharge curve. Several electrochemical studies were conducted to verify these effects of THF on LICs. The open circuit potentials of AC/Li beaker-cells with THF were shifted to more positive potentials because of the lowered electrostatic potential of the electrolyte, the weakened solvation energy with LiBF4, and the easier adsorption of BF4- ions to AC electrode in aging step. Although the concentration of LiBF4 decreased with adding THF, the electrolyte conductivity increased by the improved mobility of ions more than enough to compensate to the electrolyte conductivity decreasing by the reduced salt concentration. The capacitive resistance of activated carbon (AC)/Li cell was little changed by adding THF in fully charged state but was much smaller in fully discharged state than the reference condition. These results imply that THF seldom disturbed the behavior of BF4- ions but accelerated the mobility of Li+ ions at AC electrode. (C) 2017 Elsevier Ltd. All rights reserved.