Whey proteins (WPs) and the exopolysaccharide B40 (EPS B40) form electrostatic complexes under specific conditions. EPS B40 is a natural thickener in yogurt-like products. It is a phosphated polysaccharide and thus has a strong polyelectrolyte character. When the WP and the EPS B40 were mixed at pH values near or below the isoelectric point (pI) of the protein, soluble complexes were formed at pH, and phase separation took place below pH(phi). The formation and the structure of those complexes were studied by various methods, including turbidity, dynamic and static light scattering (DLS and SLS), and viscosity measurements. The results showed that the strength of the interaction was strongly pH- and salt-dependent. The zeta-potential of the protein at pH(c) and pH(phi) was linearly dependent on the square root of the ionic strength (rootI), showing the electrostatic nature of the interaction. Light scattering and viscosity measurements provided new results on the behavior of the complexes at the molecular level. In the region where the complexes were still soluble and at low ionic strength, the DLS radius measured in the WP/EPS B40 mixture was smaller than the coil size in the EPS B40 solution but the apparent molar mass was increased. The increase of the molecular mass was attributed to the complexation of WP on the EPS B40 chain, which, at low salt, induced a reduction of the intramolecular repulsion and led to the compaction of the polysaccharide. Also, the ratio of protein to polysaccharide was varied in order to get more insight into the dynamics, the structure, and the apparent stoichiometry of the EPS B40/WP complexes. The results illustrated that phase separation was a consequence of charge neutralization of the complexes and that the apparent stoichiometry of the complexes depends on the order of mixing of the compounds. In time. the complexes rearranged to form neutralized complexes and free EPS B40. The concept ofcooperative binding was highlighted in the case studied.