The effect of poly(ethylene glycol) on the flow behavior of Ca-bentonite and soda activated bentonite dispersions was characterized by the plastic viscosity, the relative viscosity and the yield values. In the range 0.025-12.5 mol/m(3), poly(ethylene glycol) (PEG400, PEG3000, PEG8000) was added to bentonite-water systems in different concentrations. Three layered phases with different basal spacings for each Ca- and Na-bentonites were obtained, as shown by XRD studies. The maximum swelling had a basal spacing at 1.82 nm for both PEG400 containing Ca- and Na-bentonites. In addition, expanded basal spacings are displayed at 1.91 nm for Ca- and 2.08 nm for PEG8000 containing Na-bentonites. After the PEG addition, some significant changes in rheological properties were observed; corresponds changes were also found during XRD and IR studies. The results are interpreted according to the type of bentonites, molecular weight of PEG and their concentrations. (C) 2005 Springer Science + Business Media, Inc.