The effect of halide ion concentration on the capacitor performance was considered during this study. Iodide anion has been selected as the most profitable halide taking into account its electrochemical properties and environmental impact. Several concentrations of NaI were tested (from 0.25 to 5 mol L-1 aqueous solutions) using as electrodes two commercial activated carbons and one KOH-activated carbon. Detailed electrochemical investigation by galvanostatic charging/discharging, cyclic voltammetry, and impedance spectroscopy confirmed the significant impact of iodide concentration on the supercapacitor behavior. The higher concentration of iodide affected especially the performance of positive electrode; increase of iodide concentration changed the potential range of positive electrode and its capacitance increased from 119 F g(-1) for 0.25 mol L-1 NaI to 475 F g(-1) for 2 mol L-1 NaI solution. The electrode capacitance measured in two-electrode system at current density of 2 A g(-1) ranged from 198 F g(-1) for 0.25 mol L-1 NaI to 272 F g(-1) for 2 mol L-1 NaI solution (capacitance expressed as average of the positive and negative electrode capacitances). It has been proved that 2 mol L-1 alkali metal iodide solution is an optimal electrolyte for the capacitor based on KOH-activated carbon. High capacitance values and perfect stability (100 % retention) of such systems have been observed during long-term galvanostatic charging/discharging (15,000 cycles). In addition, satisfactory floating tests at extended voltage range (1.2 V) were performed.