In this study the formation of water-soluble and water-insoluble complexes between bovine serum albumin (BSA) and biocompatible poly(N-vinyl-2-pyrrolidone-co-acrylic acid) (P(VP-co-AA); 2: 1), anionic polymer, was studied at different protein, polymer concentrations and pH values. BSAP(VP-co-AA) complexes were studied by turbidimetric, gravimetric and chromatographic methods. Stability of the protein-polyelectrolyte complexes is weak in pH > pI (isoelectric pH) when the pH value decreases (pH < pI) the stability increase and the stable water-soluble and water-insoluble protein-polyelectrolyte complexes occur. This work represents that the BSA-P(VP-co-AA) complexes were formed by the electrostatic interaction between oppositely charged BSA and P(VP-co-AA) molecules depending on variety of parameters, including the system pH, protein/polymer or polymer/protein molar ratio. BSA-P(VP-co-AA) complexes are water-soluble at pH 5.0. The system transfers to water-insoluble form with decreasing pH value. These water-insoluble protein-polyelectrolyte complexes were transformed into soluble form by increasing protein concentration. Therefore, such P(VP-co-AA)-BSA complex systems may be candidate in development of medical diagnostic, therapeutic drugs, enzyme immobilization, protein purification and biological sensors.