The rheological parameters of a clay suspension can be used to evaluate particle-particle interactions. Although there have been many studies of the rheological behavior of suspensions of reference clays, only few studies were performed with suspensions of mixed clays, and the rheological properties of these suspensions are not yet fully understood. The length-to-width ratio of some soil clays was evaluated by Egashira and Matsumoto (1981) based on viscosity measurements of very dilute suspensions of these clays. The influence of montmorillonite additions on the rheological behavior of kaolinite suspensions was determined by Keren (1989, 1991). The particle arrangement in three soil-clay suspensions was studied by Zhao et al. (1991) based on rheological measurements. The effects of mineralogical composition and particle size on the rheological behavior of concentrated suspensions of lateritic soil clays were investigated by Cerpa et al. (1999). The rheological properties of suspensions of reference palygorskite clays were investigated in a previous study (Neaman and Singer, 2000). To determine if the rheology of palygorskite suspension is affected by the addition of montmorillonite, we investigated the rheological properties of palygorskite-montmorillonite suspensions. Three palygorskite samples were used for this study: clays from Mt. Flinders (Queensland, Australia), Mt. Grainger (South Australia), and Yucatan (Sacalum, Mexico). Wyoming montmorillonite (SWy-1, Crook Country, USA) was used for preparation of palygorskite-montmorillonite mixtures. The clays were ultrasonically dispersed in water and the <2-μm size fraction was separated by sedimentation under gravity. The clays were then lightly treated by 0.1 N HCl (sample:solution ratio of 1:10) for the removal of carbonates. The samples were rapidly heated in the acid to boiling and the pH values of the suspensions were immediately adjusted to pH 7 by addition of 0.1 N NaOH. The sample from Mt. Grainger contains considerable amounts of free iron oxides which were extracted by …