The effects of cationic tetradecytrimethylammonium bromide (TDTMABr), anionic sodium tetradecyl sulfonate (NaTDS) and nonionic Brij 30 surfactants on the electrokinetic and theological properties of Na-bentonite and sepiolite suspensions have been investigated. It has been found from the zeta potential measurements that the negative surface charge of Na-bentonite and sepiolite has become positive in the presence of 7.5 x 10(-3) M and 1.0 x 10(-3) M TDTMABr, respectively. As the concentration of NaTDS is increased in the suspension, the zeta potential value of Na-bentonite does not change significantly, but that of sepiolite decreases gradually. Na-bentonite-surfactant suspensions show a thixotropic behavior, on the other hand sepiolite-surfactant suspensions display a rheopexic or thixotropic behavior depending on the surfactant concentration. All clay-surfactant suspensions follow a Bingham type flow behavior. The surfactant concentration leads to significant changes in the plastic viscosity (eta(pl)) values of sepiolite-surfactant suspensions, on the contrary of Na-bentonite-surfactant systems. Some shifts in the peak positions have been observed in the FTIR spectra of samples, which have provided evidence about the interactions between clay particles and surfactant molecules. This study shows that the suspensions having the desired zeta potential and values and thixotropic or rheopexic behaviors can be obtained using different clay-surfactant mixtures. (C) 2012 Elsevier B.V. All rights reserved.