We studied the adsorption of a cationic surfactant, dodecyltrimethyl ammonium bromide (DTABr), onto laponite in water-methanol and water-DMSO mixtures at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA(+) and Br--selective electrodes. All of the isotherm shapes in the mixture were classical. Activity coefficients were determined for the transfer between water and hydroorganic mixtures. Comparison of the adsorption isotherms obtained for different surfactant chemical potentials showed that at higher concentrations of free surfactant, the aggregation state does not depend on the nature of the surrounding media. A study of the influence of the ionic strength showed that there is a common intersection point in each hydroorganic mixture. This point corresponds to the charge compensation point and depends on the nature of the surrounding medium. (C) 2003 Elsevier Inc. All rights reserved.