PURPOSE: To provide the titled compsn. which can be easily molded, is not discolored during heating and is suitable for use in the production of gas barrier film, etc., by blending an organotin compd. and a dimethylformamide/boron trifluoride complex with polyacrylonitrile.

CONSTITUTION: 1W10pts.wt. organotin compd. (A) such as monomethyltin triacetate or monomethyltin trithioglycolate 2-ethylhexylate mercaptide and 1W10pts. wt. dimethylformamide/boron trifluoride complex (B) are mixed with 100pts.wt. polyacrylonitrile (C) to obtain the desired molding polyacrylonitrile compsn. By this blending, the intermolecular cohesive force of polyacrylonitrile is relaxed to thereby facilitate molding and further polyacrylonitrile can be prevented from being discolored by an intramolecular cyclyzation reaction during heating. Component C can be obtd. by adding an equimolar amount of boron trifluoride to dimethylformamide.