The yield stress and zeta potential (sigma) of slurries prepared from as-received goethite were evaluated as a function of pH, solids concentration, cationic polyethylenimine (PEI) concentration and molecular weight (Mw). The goethite slurries displayed a maximum yield stress at the isoelectric point (pl). These slurries do not have a low pH dispersed region. The yield stress obeyed the yield stress-DLVO force model. The maximum yield stress displayed a dependence on the solids volume fraction to a power of 3.8. PEI adsorbed and changed both the surface chemistry and rheological yield stress of the slurries. At most additive concentration ranging from 0.1 to 0.4dwb% for PEI of Mw 600,1800 and 70,000, the maximum yield stress of the slurries is larger than that with no adsorbed additives. At low PEI concentration of 0.1 dwb%, the same maximum yield stress of 30 Pa was obtained for three Mws: 600,1800 and 70,000 representing a 1.5-fold increase. The increased attraction between particles was attributed to charge patch attraction at low surface coverages and hydrogen bonding at high coverages. Adsorbed PEI decreased the yield stress at low pH. However complete dispersion was observed only for 0.4dwb% PEI with Mw 70,000, at pH below 7. Complete dispersion was not observed at high pH for the same polymer at all concentrations. (C) 2009 Elsevier B.V. All rights reserved.