The effect of anionic polyelectrolytes on the electrokinetic and rheological properties of concentrated Si3N4 suspensions was investigated experimentally. We have compared novel anionic methacrylic acid comb copolymer modifications with grafted poly(ethylene oxide) chains, PMMA-PEO, with poly(acrylic acid) (PAA); the rheological response as a function of pH was very similar, thus illustrating that the grafted PEO chains have a minor influence on the colloidal stability. The polyelectrolyte adsorption and, thus, colloidal stability at pH> is mainly governed by the surface charge of the solid phase. We found that the colloidal stability and the steady shear rheology of the Si3N4 suspensions with addition of anionic polyelectrolytes could be generalized using a division into four different pH regimes, following a previous study by Hackley. The viscosity minimum around pH 7 was attributed to electrosteric stabilization. The effect of excess addition of polyelectrolyte was also discussed; we attribute the significant increase in viscosity to the ionic strength increase caused by the release of the associated counterions.