The zeta-potential of monodisperse polystyrene latices as a function of ionic strength is reported. Latices exhibit anomalous electrokinetic behavior : instead of the expected decrease in mobility with increasing electrolyte concentration, a maximum occurs. In this paper it is shown that both in electrophoresis of dilute dispersions and in electroomosis of concentrated plugs, the zeta-potential as a function of ionic strength passes through approximately the same maximum at about the same electrolyte concentration. After the correction for the influence of conductance by ions in the diffuse electrical double layer, using O＇Briens model, still a slight maximum exists. The results provided an estimate of the Stern layer thickness. An analysis using the hairy layer model, when surface conductance was neglected, provided an estimate of the hairy layer thickness.