This work is devoted to the study of the rheological properties of sodium montmorillonite suspensions in aqueous media containing polyelectrolytes in solution. Two different polyelectrolytes are employed: polyacrylic acid (PAA) and polyethyleneimine (PEI). PAA can bear negative charge, thus acting as a polyanion, while PEI can be considered as a polycation, although the charge of both polymers is strongly dependent on pH of the solution. The theological behavior of clay suspensions is essentially determined by the electric potential of the faces and edges of the laminar clay particles. In order to analyze the changes in the interfacial electric potential of clay surfaces, the zeta potential of clay particles was estimated from electrophoresis measurements for different solution compositions. The yield stress and the storage modulus of the suspensions were determined demonstrating that only in some cases the storage modulus can be correlated with the changes in electrostatic interactions between particles. In particular, in clay/PEI suspensions at neutral-basic pH the changes in the viscoelastic properties do not match with those in surface-to-surface electrostatic interactions. Different mechanisms are proposed to explain the wide variety of rheological phenomena observed. (c) 2006 The Society of Rheology.