The electrochemical properties of 1-ethyl-3-methylimidazolium difluorophosphate (EMImPO(2)F(2)) ionic liquid have been investigated as an electrolyte for electric double-layer capacitors using activated carbon electrodes. A two-electrode cell test reveals that the capacitance of EMImPO(2)F(2) exhibits a larger voltage dependence than that of a typical ionic liquid electrolyte EMImBF(4). At a charging voltage of 2.5 V, the capacitance obtained for EMImPO(2)F(2) is 49 F g(-1) and is larger than 44 F g(-1) obtained for EMImBF(4). According to the charge-discharge cycle test, the breaking-up voltage of EMImPO(2)F(2) is lower than that of EMImBF(4), which may be caused by the lower anodic stability of PO2F2- than that of BF4-. The ionization potential of PO2F2- was calculated by quantum mechanical calculations to estimate its anodic stability and was compared with those of several anions used in typical electrolytes. The results indicate that the anodic stability of PO2F2- is similar to that of ClO4- and lower than those of BF4- and PF6-.