AbstractComplex formation in poly(vinylpyrrolidone)‐poly(itaconic acid monomethylester) and poly(vinylpyrrolidone)‐copolymer of maleic anhydride and monomethylitaconate systems has been investigated. It is shown that the interpolymer complexes are formed at interaction of macromolecular components. Data of conductometric, potentiometric, and viscometric titrations indicate that the molar composition of complexes is PVP:PMMI = 1:1 and PVP:MA/ MMI = 3:2. The complex particles are very compact with the intrinsic viscosity in water about 0.05‐0.10 dl/g not depending upon the molecular weight of polymer components. Complexes are destroyed both by inadequate organic solvent (30‐40 vol %) in mixed solvent water ‐dimethylsulfoxide, and by therm