Counterion polarization of sodium and magnesium polystyrenesulfonate (NaPSS and MgPSS), adsorbed in an excess onto ellipsoidal beta-ferric hydrous oxide particles (beta-FeOOH), is studied in both the absence and the presence of added simple electrolytes, NaCl and MgCl2. The amplitude of the low-frequency (10(2)-10(4) Hz) and the high-frequency (10(4)-10(6) Hz) electro-optical effects of NaPSS is found to decrease with increasing NaCl concentration, while the critical frequency of relaxation of both effects remains unchanged. Addition of MgCl2 to the suspension containing MgPSS also reduces the amplitude of the low-frequency electrooptical effect, but the amplitude of the high-frequency effect increases, The increased number of mobile (not bound) divalent ions, which produce larger charge fluctuation, could explain the high-frequency effect increase in the presence of a small excess of MgCl2. Substitution of Na+ counterions of PSS by Mg2+ is found to increase the critical frequency of relaxation and to reduce the amplitude of the low-frequency effect, probably due to the strong repulsion between bound divalent counterions. The polyion length remains nearly unchanged during this substitution, as evidenced by the constant relaxation time of the polyelectrolyte-coated particles. The increase of counterion valence is not found to affect the amplitude of the high-frequency electro-optical effect in the absence of added salt.