The invention provides a method, and apparatus, for determining the degree of separation (DOS) of a polymer solution into a polymer-rich stream and a solvent-rich stream, said method comprising the following: adding to a liquid-liquid separation vessel the polymer solution, which comprises a polymer, a solvent and an anti-solvent; separating the polymer solution into a polymer-rich stream and a solvent-rich stream; removing at least some the polymer-rich stream from at least one outlet P on the vessel, and measuring the actual solution density of this polymer-rich stream using at least one flow meter; removing at least some of the solvent-rich stream from at least one other outlet S on the vessel, and measuring the actual density of the solvent-rich stream using at least one flow meter; and wherein the degree of separation (DOS) is determined by the following equation (Eqn. 1): DOS=[actual solution density (polymer-rich steam)−actual solution density (solvent-rich stream)]/[theoretical solution density (polymer-rich stream)−theoretical solution density (solvent-rich stream)]  (Eqn. 1).