Measurements have been done of the effective diffusion coefficient versus ionic strength (KBr) of isolated monodisperse polystyrene particles (approximate to 200 nm in diameter) with different superficial groups, sulfonated and carboxylic, via photon correlation spectroscopy (PCS). The sulfonated latexes behave as could be expected according to the DLVO theory of colloidal stability. However, the carboxylic latexes exhibit a stabilization at ionic strengths higher than their c.c.c. Although the hairy layer model can explain the results below the c.c.c. it cannot explain restabilization. Due to both the charge and the hydrophilic character of our carboxylic latexes, we suggest the possible existence of a hydration force at high salt concentration. This would give rise to a repulsive interaction energy between particles that could be responsible for stabilization of these systems at high ionic strengths.