The relationship between saturated adsorption amount (A(s)) and pH and the relationship between saturated adsorption amount (A(s)) and zeta potentials for two cationic dyes and three anionic dyes on activated carbon have been studied. It has been found that A(s) of the dyes on activated carbon is correlated with the electrostatic forces between charges on the carbon surface and ionic dyes. The experimental results showed three distinct states regarding the effect of zeta potential on carbon surface. In the neutral potential state where zeta potential is nearly zero, the electrostatic interactions between the carbon surface and the dyes are negligible and the adsorption forces are mainly dispersion forces. Therefore, pH has no effect on A(s). In the positive or negative potential states, there exists the electrostatic attractive or repulsive forces between activated carbon and the ionic dyes. The adsorption forces are the sum or difference of dispersion and electrostatic forces. The adsorption amount, A(s), changes with zeta potential. The different types of adsorption forces in the three potential states reveal the mechanism of adsorption for the dyes on activated carbon. The conclusion is further supported by the kinetic and thermodynamic parameters that were calculated from the experimental data.