in this work we analyze the rheological behavior of Na-montmorillonite (NaMt) suspensions in the presence of humic acid. The analysis starts from the fact that the electric charges on plate and edge surfaces of clay particles largely determine the formation of three-dimensional structures in suspensions. Zeta potential data of NaMt as a function of humic acid HA) concentration suggest that its adsorption takes place preferentially on the edges of the particles. A good correlation between the parameters describing the viscosity-shear rate or viscosity-shear stress dependencies and the product of the; potentials of edges and faces, edge) face is demonstrated. It indicates that the rheological behavior of NaMt suspensions is largely controlled by electrostatic interactions, Adsorption of HA also changes the viscoelastic properties of the suspensions significantly as determined by both oscillometric and creep-recovery tests. A trend of NaMt suspensions from approximately solid like when no HA is adsorbed, to almost liquid like when the HA concentration is in the range of 50-100 mg/l is observed.