ACF discs were surface treated using NaOH, HNO₃ and connected to a graphite bar, for the electrosorption of cobalt ions from an aqueous solution. The total acidity of ACF decreased by NaOH treatment which maintaining the same average pore size and specific surface area. The total acidity, specially carboxyl groups, increased by HNO₃ treatment, although the specific surface area was reduced by co. 20%. The Point of Zero Charge (PZC) of ACF decreased from pH 3.1 to 2.6 and 2.2 by HNO₃ and NaOH treatment, as a result of increases in anode ion density on ACF disc surface. Therefore, the adsorption rate of cabalt ion by NaOH treated ACF disc was twice larger than that of non-treated ACF disc, while that of HNO₃ treated ACF disc was only one fifth of the non-treated ACF disc in electrosorption. The electrosorption rate increased with the electrolyte concentration, however, it slowly decreased above 0.2 N NaCl because of competition between Co²⁺ and Na⁺.