Abstract: The efforts of adsorption of alkyltrimethyl ammonium bromides on the electrophoretic mobility of a monodisperse polystyrene latex were examined experimentally.The adsorption isotherms were obtained from the electrophoretic data in terms of Stern electrical double layer model.An improved method was proposed for evaluating the number of adsorption sites and adsorption free energy,and the possible mechanism of adsorption was discussed.