The effect of hydrolyzable metal cations, i.e. calcium and magnesium, present in mineral suspensions on starch-kaolinite interactions was studied through zeta potential, adsorption and turbidity measurements. The results demonstrate that hydrolyzable metal cations in mineral suspensions can significantly affect starch adsorption on kaolinite under typical conditions of iron ore flotation and selective flocculation. The effect of calcium and magnesium on starch adsorption on kaolinite surfaces correlates to their abilities to reduce the electrostatic repulsion between the anionic starch molecules and the negatively charged kaolinite particles. The hydrolyzable metal cations cause fast coagulation of kaolinite particles at pH 10.5 by reducing the repulsive electrostatic forces between kaolinite particles, when their concentration is >= 0.001 M. The presence of starch further enhances such effect at low dosages, but re-disperses kaolinite particles at high dosages by inducing steric forces between the kaolinite particles coated with starch. (C) 2010 Elsevier B.V. All rights reserved.