The resistance of aqueous latex dispersions to removal of water has been measured through osmotic compression. For pure polystyrene particles stabilized with sulfate groups anchored at their surfaces, the measured pressures match the predictions of models for the interactions of charged spheres in water. For copolymer particles (styrene-butyl acrylate) covered with acrylic acid sequences linked to the core polymers, the measured pressures are considerably higher than the predictions. This strong resistance to dehydration originates from polyelectrolytes which are released by the particles or extend from their surfaces into the aqueous phase. Consequently the behavior of such dispersions is determined by the content of the aqueous phase that separates the particles.