The effects of pH and aluminium sulphate concentration on the stability of whey suspension have been investigated. The particle size and particle size distribution are examined by scanning electron microscopy and granulometric analysis, respectively. The zeta potential and residual turbidity of 0.01 wt% whey suspensions are determined over the pH range of 3-8 with aluminium sulphate as background electrolyte. The results show that the zeta potential of whey suspensions depends on pH and, to a lesser degree, aluminium sulphate concentration. The average particle size of flocs in suspension increases with decreasing zeta potentials which is accompanied by low turbidity. The degree of flocculation of whey suspension is linked to the zeta potential. The DLVO calculations are in good agreement with the observed trends indicating that the classical DLVO theory can be used to predict colloidal stability of the whey protein suspensions in the presence of higher strength electrolyte and varying pHs. (C) 2009 Elsevier B.V. All rights reserved.