Abstract Interaction of polyacrylic acid (PAA) with bovine serum albumin (BSA) at different pH values and in a wide range of mixing molar ratios, γ = n BSA/ n PAA, of components was investigated by size-exclusion high performance liquid chromatography with on-line refractive index, UV, light scattering and viscometer detectors. The results revealed the formation of stable water-soluble polymer–protein complexes at pH 5.0. For the soluble complexes thus formed, the number of the bound BSA molecules with one PAA molecule was expressed by a Langmuir-type equation as a function of the amount of excess BSA existing free in the solution. At saturation, one BSA molecule is bound to about 48 acrylic acid residues. The γ-dependencies of molecular properties and structural parameters (molecular weights, molecular-weight distribution, radius of gyration, and the Mark–Houwink equation constants) of aqueous solutions of polycomplex particles have been studied. It has been concluded from these results that the complex molecule is formed by the molecular association–dissociation processes between particles depending on protein molecules in mixtures. We assume that side-by-side association of BSA–PAA complex particles took place at γ ⩽ 5. At γ > 5, dissociation of the aggregates occurred by the including certain protein molecules into composition and by the compactization of polycomplex particles.