The role played by carbon porosity and electrolyte chemistry in the development of double-layer supercapacitors based on solvent-free ionic liquids (ILs) of a wide electrochemical stability window is investigated. Voltammetric studies performed in N-methyl-N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide, and N-methyl-N-butyl-pyrrolidinium tris(pentafluoroethyl)trifluorophosphate ionic liquids and PYR14TFSI-tetraethyl ammonium bis(trifluoromethanesulfonyl)imide solutions demonstrate that the pore-to-ion size ratio and the porous electrode/IL interface properties may have a higher impact on the electrode electrical response than do the inherent IL bulk properties. The effect of carbon porosity on the electrode capacitance and charge storage capability in both the positive and negative potential domains is discussed in relation to the IL properties, and an estimation of the upper limits of the performance of IL based supercapacitors with carbons of optimized porosity is reported. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3514694] All rights reserved.