Polystyrene latices were prepared by the emulsion polymerization method using hydrogen peroxide as an initiator and sodium dodecyl sulfate as the emulsifying agent. Latices of different particle sizes were prepared by the seeded growth technique. The particle size and its frequency distribution were determined by electron microscope. The viscosities and electrophoretic mobilities of the latex and latex:added electrolyte mixtures were measured in the presence of gradually increasing concentrations of quaternary ammonium counterions of different ionic sizes. The ζ-potentials were calculated by the computational method of Wiersema-Loeb-Overbeek (WLO); the surface charge densities σ were calculated by the method of Loeb-Wiersema-Overbeek (LWO). The primary electroviscous effects were calculated by using the Booth equation. The free energies of adsorption ΔḠ(ads), of the counterions on the particle surface were calculated. The stability characteristic of the latices and latex-added electrolyte mixtures has been studied on the basis of the DLVO theory.