The phase behavior of the polyanion-polycation system, consisting of a branched polyethylene imine and a sodium salt of polyacrylic acid, was investigated as a function of the polymer concentration. At low polymer concentrations a small area of initial phase separation was observed, surrounded by a homogeneous one-phase system. The homogeneous, concentrated polyanion-polycation systems were characterized viscometrically, densitometrically, and spectroscopically. Under special mixing conditions, a significant viscosity enhancement was observed due to Coulombic interactions between the oppositely charged polyelectrolyte components. The interactions of the polyelectrolyte components with kaolin were investigated separately in the concentration range of initial phase separation as well as in the area of concentrated, optically clear polyanionpolycation systems. In the concentration range of phase separation polyanion-polycation interactions are dominant with regard to polyelectrolyte-kaolin interactions. When adding concentrated homogeneous polyanion-polycation systems to concentrated kaolin dispersions above a critical polyelectrolyte concentration, a transition to polyelectrolyte-kaolin pastes was observed. In general, these results are of practical importance for a better understanding of structuring phenomena in pigment systems.