The adsorption isotherms of dodecyltrimethylammonium ions (DTA(+)) adsorbed onto a laponite clay colloid were studied in water and in NaBr solution (at 0.1, 0.3, and 0.5 mol . liter(-1)), at 298 K, using amphiphilic ion-specific electrodes, This potentiometric method can be used to measure concentrations in situ and to follow the decrease in the amount of surfactant in the solution as the surfactant is adsorbed onto the day, This paper describes a method for characterizing adsorption isotherms of DTA(+) onto laponite by determining the amount adsorbed at a fired surfactant activity in solution. By repeating this process for various DTA(+) activities, the adsorption isotherm can be plotted, Adsorption isotherms obtained with or without NaBr in the solution were continuous curves which increased with the surfactant activity. The field of study is limited by the CMC value for the surfactant and the curves have a common intersection point, which can be attributed to the isoelectrical point of the laponite/adsorbed surfactant system, This method can be generalized to other cations and obviously to other solids.