Several commercially available separators as well as novel ultraporous electrospun nonwovens were evaluated in mild aqueous supercapacitor cells, using conventional electrochemical techniques. A special laboratory test-cell was designed and fabricated to accurately control the pressure applied to the stack. It was shown that the separator layer not only affects the equivalent series resistance (ESR) of the cell (high frequency range) but has also an influence on its time constant (medium frequency range). Both ESR and time constant were found to be directly proportional to the separator thickness. Higher porosity and higher pore size were found to lead to better performance, especially at higher power demand. The electrospun separators outperformed the other separator structures in a disruptive manner, most probably due to a synergetic effect of higher porosity and higher pore size. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.020207jes] All rights reserved.