Polycomplex formation of bovine serum albumin (BSA) and polyacrylic acid (PAA) was studied by pH titration, fluorescence, and HPLC methods in water solutions. It was shown that the complex formation and the solubility (phase state) of the polycomplexes depended on PH of the solutions and ratio of the components. The stability of PAA-BSA complexes was negligibly weak when pH = 6.0-7.0 [pH > pI (isoelectric pH)]. Stable water-soluble polycomplexes formed at pH = pI (PH 5.0) in a wide range of n(BSA)/n(PAA) ratios (0.05-50) and coexisted with free protein molecules at the higher ratios of components. Existence of water-soluble and insoluble PAA-BSA complexes has been observed at pH <= 4.5. The soluble to insoluble state transition of polycomplexes has been confirmed and binding mode of polyelec-trolytes to proteins was assumed dependent on the ratio of components as a result of formation of soluble polycomplexes, complex coacervation, or amorphous precipitates. Soluble complexes were formed as fully homogenized mixtures. Fraction composition of the mixtures and insoluble complexes depended on the protein/polymer ratio and over the critical protein/polymer ratio, the soluble polycomplexes coexisted with free BSA molecules. On the other hand, the comparision of covalent conjugate formation and complexation of PAA with BSA has been revealed. (C) 2007 Wiley Periodicals, Inc.