The effect of cationic surfactant dodecyltrimethylammonium bromide (DTABr) on the flow behavior of Na activated bentonite dispersions was characterized by plastic viscosity, relative viscosity and yield values. In order to make definite the type of flow behavior, three common empirical equations were tested: Bingham, Power law and Casson equations. The constants for the three models were determined by using least square approximation and in each case the coefficient of determination (r(2)) was calculated. The interaction of the surfactant with external surfaces of clay was investigated by electrokinetic measurements. Whether DTABr molecules penetrate into interlayers of clay structures or not can be determined on basis of d((001))-spacing measurements using XRD after addition of each additives. All experimental results were evaluated by considering the interaction modes between the particles and surfactant molecules. (c) 2005 Elsevier B.V. All rights reserved.