Abstract: From the electrophoretic and conductrometric measurements it was deduced that the increase of zeta potential of PS latex with increasing concentration of electrolytes was mainly due to the ion exchange occurred between the surface groups on the latex particle and the cations in the solution. According to the Langmuir's equation of adsorption and Stern's electrical double layer model, the adsorption free energy and number of the adsorption sites of surface active cations on PS latex were evaluated from electrophoretic data. Increasing the electrolyte concentration enhanced the number of adsorption sites of latex particle and hence the adsorption amount of surface active cations.