This invention relates to a stabilized solution of a copolymer from acrylonitrile and vinylidene chloride in an organic solvent, the chlorine content of the copolymer is in the range from 20 to 60 percent by weight based on the polymer. The solution of the copolymer contains a salt of a mercapto sulphonic acid with a guanidine as stabilizing agent, said salt is present in the solvent in the range of from 0.001 to 5 percent by weight based on the polymer.