Multiwalled carbon nanotube (MWCNT) based electrical double-layer capacitors (EDLCs) using ionic liquid incorporated magnesium ion conducting gel polymer electrolytes were constructed and characterized. For comparison, three electrolytes were used to fabricate EDLCs comprised of a room-temperature ionic liquid 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate (EMITf) or a solution of magnesium salt (magnesium trifluoromethanesulfonate or Mg triflate) in EMITf or a solution of magnesium salt in the mixture of EMITf and a cosolvent of ethylene carbonate (EC): propylene carbonate (PC), immobilized with poly(vinylidene fluoride-hexafluoropropylene). The comparative performance characteristics of the EDLCs based on impedance analysis, cyclic voltammetry, and galvanostatic charge-discharge tests for numerous cycles were presented. The role of magnesium ions and the EC:PC cosolvent incorporated in the gel electrolytes was important in the dramatic improvement of the EDLCs' performance. The optimum capacitance value of 64-82 mF cm(-2) (equivalent to the single electrode specific capacitance of 32-41 F g(-1) of MWCNTs) was achieved for the EC: PC and the ionic liquid incorporated gel electrolyte based EDLCs. This corresponded to the specific energy of similar to 17 Wh kg(-1) and the high value of power density of similar to 13 kW kg(-1). The EDLCs showed excellent cyclic performances for 5 x 10(4) cycles or even more. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3258317] All rights reserved.