Carbon xerogel (CX) materials were synthesized via polycondensation reactions on a carbon cloth sheet, and the resulting sheets were used as electrodes in a capacitive deionization (CDI) cell. To study the effect of oxidation degree for the CX on the electrosorption performance, the electrodes were activated in concentrated HNO3 at various temperatures. Electrosorption examined in a flow cell shows that the electrosorption capacity significantly increases from 3.27 mg (NaCl) g-1 (Carbon) (corresponding to the untreated sample) to 5.46 mg (NaCl) g-1 (Carbon) (for the sample treated at 20°C). However, this value cannot be further improved by adding additional functional groups to the carbon surface by treating the sample at higher temperatures. These additional functional groups may interfere with the double layer used for ion adsorption. Finally, we conclude that the CX materials mildly treated in HNO3 could be a way of greatly improving the deionization capacity of CX for CDI applications.