The objective of this work is to study the flocculation of Ca-and K-montmorillonite in the presence of some coagulant (CaCl2 and KCl) and mixtures of coagulants and polyethylene oxide (600,000 mol wt) through measuring the zeta potential. The action of Ca2+ was more effective than K+ in decreasing the zeta potential of Ca-montmorillonite; consequently, the electrical double layer of the clay particles has been reduced. On the other hand, the action of K+ was more pronounced than Ca2+ in decreasing the zeta potential of K-montmorillonite. This contradicting result was attributed to the change of the partial molal volume of Ca- and K-montmorillonite. The flocculation of Ca- and K-montmorillonite by polyethylene oxide was enhanced when the clay was initially treated with electrolyte. This electrolyte reduces the thickness of the electrical double layer. Under this condition, the polymer segments collapse on the clay surfaces, leaving polymer loops and tails available for contacting adjacent clay particles.