The effects of ionic copolymers with hydrophobic segments on the yield stress-pH behavior of ZrO2 dispersions were evaluated. These polyelectrolytes were two alternating copolymers; diisobutylene-maleic acid and a-methyl styrene-maleic acid, and a random styrene-maleic acid copolymer. At low concentrations, all three copolymers caused a much greater reduction in the maximum yield stress, tau(ymax) (at the neutral charge condition), compared with hydrophilic polyacrylic acid and acrylic acid-ester polymers. The reduction in tau(ymax) was as much as 75%. The much larger reduction in tau(ymax) was attributed to a thicker steric layer formed by the ionic copolymers with hydrophobic segments. At higher concentrations, all three copolymers caused tau(ymax) to increase. This was attributed to significant hydrophobic effects and attraction. The degree of hydrophobic segment aggregation at the closest point of-interaction between particles is now higher. This also caused an overall increase in the entropy relative to the state when the particles are far apart, as the degree of hydrogen bonding among water molecules is less when the hydrophobic segments are aggregated. (C) 2003 The Society of Rheology.