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