The effects of hydrolyzable Cu2+, Co2+, Ni2+ and Al3+ ions on the zeta potential and yield stress behaviour of ZrO2 and SiO2 dispersions were evaluated as a function of pH. With ZrO2 dispersion, adsorbed hydrolysis product of Cu2+ increases the yield stress at zero zeta potential while that of Al(III) decreases it. As the particles are completely coated with hydrolysis product, the most probable explanation for the stronger attraction between ZrO2 particles coated with Cu2+ hydrolysis products must be due to an increase the effective Hamaker constant of particle in water. In the case of ZrO2 particles coated with Al3+ hydrolysis products, the effective Hamaker constant is decreased. Silica dispersions are not flocculated even at its point of zero charge of pH similar to 2.0. However this dispersion is flocculated by the adsorbed hydrolysis products of Co2+ and Ni2+ ions. The onset of flocculation corresponded to the pH of formation of the hydrolysis products. With silica dispersions, at low coverage of hydrolysis product, the attractive forces contributing to the yield stress are charged patch and bridging. At high surface coverage at the pH of maximum yield stress, the increased in the effective Hamaker constant by the hydrolysis product is responsible for the enhanced van der Waals attraction. (C) 2006 Elsevier B.V. All rights reserved.