In order to obtain well dispersed concentrated aqueous Si3N4 slips for slip casting, the influence of pH and sintering aid powders (Y2O3 and Al2O3) on the viscosity and on the amount of tetramethylammonium ions adsorbed were determined. 35 vol% aqueous Si3N4 and Si3N4-6wt%Y2O3-4wt% Al2O3 slips were prepared in an attrition mill. Tetramethylammonium hydroxide was added to adjust the pH values in a range of 9.7 to 12.3. The viscosity of Si3N4 slips and the amount of [(CH3)4N]+ ions in solution increased with increasing pH. These counterions contributed mainly to increase the ionic strength of the solution with increasing the slip viscosity. The sintering aid powders had a positive effect on the dispersion of the Si3N4 powder at pH 10.3-12.3 since low viscosity values could be obtained. This was attributed to the lower concentration of counterions in solution.