In this paper the electrophoretic properties of large monodisperse polystyrene (PS) latices, prepared by a one-step surfactant-free emulsion polymerization, are described. The latices were characterized by conductometric titration. The charged groups, arising from initiator fragments, were all strong acid groups and no carboxyl groups could be detected. The amount of surface charge groups varies with particles size. Electrophoresis measurements have shown that the electrophoretic mobility passes through a maximum as the electrolyte concentration is increased. Converting the electrophoretic mobility to zeta-potentials, taking into account electrophoresis and retardation effects but neglecting surface conductivity gives the same picture. The same phenomenon is observed with increasing H+ concentration. Only at very high electrolyte or H+ concentrations does the zeta-potential reach values close to zero. The zeta-potentials were converted to charge densities behind the electrokinetic shear plane (sigma zeta), using the Gouy-Chapman theory. In general, PS particles behave as expected without discrepancies between experiment and theory : sigma(0) (sigma(0) is the surface charge density) is larger than sigma zeta. For one particular latex, however, a sigma zeta larger than sigma(0) was found. This was observed at high electrolyte concentrations, where uncertainties in the zeta-potential because of neglect of surface conductivity are not important. This fact was attributed to chemisorption of NO3- ions. Chemisorption of NO3- ions is not a common fact, but may be attributed to the pronounced hydrophobic character of the PS particles.