Abstract: The molecular characteristics of polyacrylic acid under conditions close to physiological are determined by dynamic light scattering, viscometry and analytical ultracentrifugation. Mark–Kuhn–Houwink equations are obtained, and the equilibrium rigidity of polyacrylic acid (PAA) and the hydrodynamic diameter of the polymer chain are determined. The process of the formation of a complex of polyacrylic acid with model oligoethylenimines: diethylenetriamine, triethylenetetramine, pentaethylenehexamine is investigated. When oligoethyleneimines are mixed with PAA-Na in a phosphate buffer a complex is formed. It is found that the solution contains particles whose hydrodynamic radii correspond to the initial polyacrylic acid, as well as large particles of the complex. The transition to acidic pH increases the proportion of large particles in the solution.