PURPOSE: To remove impurities contained in a highly viscous material easily by removing ions of the impurities from a soln. prepd. by dissolving the highly viscous material in the first solvent, then precipitating the highly viscous material with the second solvent.

CONSTITUTION: An organic material giving high viscosity to the soln. is dissolved in the first solvent. The soln. is allowed to contact with cation exchange resin to remove ions of impurities contained in said org. material. Then, the soln. is diluted with the second solvent, which does not dissolve substantially said org. material but is optionally miscible with the first solvent, to precipitate the deionized org. material. By this method, impurities in the org. material which exhibits high viscosity are removed easily.