We applied flow field flow fractionation in a ceramic tubular membrane to study the diffusional behavior of sodium polystyrene sulfonate (NaPSS) as a function of salt and polyelectrolyte concentrations in the dilute concentration regime. NaPSS standards were fractionated in an aqueous carrier liquid with salt concentrations of 1-100 mmol L-1 and injected amounts of 0.2-20 mug. A sharp increase in the apparent diffusion coefficients, due to peak overloading, occurred at a critical polyelectrolyte concentration at the accumulation wall. A model has been developed that compares repulsive electrostatic interactions, calculated from the Derjaguin-Landau-Verweij-Overbeek theory, to the thermal energy. It was found that the observed critical concentration approximated the concentration of a closely packed lattice of hard spheres within an order of magnitude. The critical concentration appeared to be dependent on the salt concentration, but the influence of the molecular mass was less clear. Expressions for the migration velocity in overloaded channels were derived and confirmed experimentally for the two lowest molecular masses. Deviations occurred for high molecular mass NaPSS, possibly because of mutual attraction. These experiments confirmed the observations made by small-angle neutron scattering and dynamic light scattering that the ordering of polyelectrolytes already occurred in the dilute regime.