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A method was developed for finding the concentration of fibre-forming polymers in spinning solutions based on extraction and subsequent titrimetric determination of the solvent. It was found that the solvent is totally extracted from the spinning solution in most cases only during titration. When an extractant is added to the solution, two related equilibria are established between polymer and solvent and between solvent and extractant. The acid-base interaction between solvent and titrant shifts the equilibrium toward extraction of the solvent from the polymer solution.