Complex formation between sodium polystyrenesulfonate (NaPSS) and two proteins, bovine serum albumin and beta-lactoglobulin, was studied by dynamic light scattering (DLS) and capillary electrophoresis (CE) over a range of polyelectrolyte molecular weights. We found that the dimensions of the polyelectrolyte chain do not appear to change significantly upon complex formation and that the dependence of complex mobility on its composition is in agreement with a free-draining model. Estimates of intrinsic binding constants, sizes of binding site, and cooperativities in complex formation were obtained by fitting experimental data to the "overlapping binding sites" model. It was found that the cooperativities shown by the binding isotherms are consistent with the dependence of binding density upon NaPSS molecular weights. The latter can be predicted by the overlapping binding sites model for known binding cooperativity, but was not observed experimentally before.