Characteristic theologic properties, such as shear stress, viscosity, yield point of Na-activated bentonite were studied after adding an anionic surfactant (Linear alkyl benzene-sulfonate, LABS) and a cationic surfactant (distearly dimethyl ammonium chloride, DDAC). The effect of the surfactants also have been investigated at different quantities. The results have been discussed considering the types of additives and their concentrations. (C) 2000 John Wiley & Sons, Inc.