Abstract--The prevailing concept that positive-edge to negative-face attraction accounts for the rheological behavior of montmoriUonite suspensions at low electrolyte concentration was investigated. In one experiment, Mg 2+ released from Na-montmorillonite was measured at several NaC1 concentrations; in a second experiment, the viscosity, n, and the extrapolated shear stress, 0, were measured at several clay concentrations, pHs, and NaC! concentrations; and in a third experiment, the absorbance, A, was measured at two wavelengths (450 and 760 nm) at different clay and electrolyte concentrations. The released Mg 2+ decreased with increasing NaC1 concentration until it became zero at a NaC1 concentration between 0.01 and 0.02 M, depending on pH. Thereafter, it increased with increasing NaC1 concentration. Both 0 and n were highly correlated with the amount of released Mg 2+. Also, A remained constant until the NaC1 concentration corresponded to that at the minimum of 0. Thereafter, it increased and became linearly related to 0. These results suggest: (1) positive-edge to negative-face interaction cannot solely account for the rheological properties of montmorillonite at low electrolyte concentration, and (2) the release of octahedral Mg 2+ from montmorillonite affects 0, because it reduces the negative charge on the particles and, thereby, the repulsive force between them.