The flat band potentials, E(FB), of naked n-WS2 electrodes and cationic viologen polymer-modified n-WS2 electrodes have been determined in KI and KI3 solutions by differential capacitance measurements. The E(FB) values for naked n-WS2 electrodes are shifted negatively in electrolyte media containing I- or I-3(-). This negative shift is much larger in the I-3(-) solution than in the I- solution. The magnitude of the shift depends on the concentration of I- or I-3(-) in the solution, and this dependence can be modeled by Langmuir adsorption isotherms. The polymer-modified electrodes are formed by deposition of the polymer via hydrolysis of N,N’-(bis(p-trimethoxysilyl)-benzyl)-4,4’-bipyridinium. Because of the high concentration of anions in the cationic viologen polymer due to electrostatic forces, the polymer-modified n-WS2 surface is in contact with a high concentration of I- or I-3(-) even in dilute I- or I-3(-) solution. However, the E(FB) shifts are about the same for both naked and polymer-modified electrodes in a wide range of concentrations of I- or I-3(-) solution, as expected from a thermodynamic analysis.