Preventing or reducing the rates of electrolyte decomposition reactions is essential for the stable long-time operation of Li-ion batteries and supercapacitors. Our study investigates the factors that control such reactions on high-surface area carbon electrodes. We show that a decrease of the ratio between the adsorbed ion size and the available carbon pore width (or IS/PW ratio), although resulting in a some loss of reversible capacitance due to the ion sieving effect, can substantially increase the electrochemical stability window (ESW) of carbon electrodes. Cyclic voltammetry and electrical ac impedance characterization of three different carbons immersed in four tetra-alkylammonium or Li+ cation-containing salts dissolved in three different aprotic solvents, and two ionic liquids, provide evidence for this correlation. The confined space in carbon nanopores can significantly extend the ESW of an electrolyte solution, while the excess electrolyte solution contained in wide pores substantially decreases the threshold for degradation processes. In a broader context, Our study indicates that the ESW, specific capacity and rate capability can reach their maximal values simultaneously, and that the IS/PW ratio provides a means for selecting a nanoporous carbon-and electrolyte solution combination which optimizes the capacity, rate capability and ESW for a given practical application. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.058304jes] All rights reserved.