The principle of the method specified is dissolving a test portion in dimethylformamide and injecting a small volume of the solution into a gas chromatograph equipped with flame ionization detector to separate and detect the volatile components. The solvent contains a known amount of propionitrile or acetonitrile as an internal standard for quantitative evaluation. With this method, a lower detection limit of the order of 3 ppm of acrylonitrile in the copolymer may be expected. To obtain a lower detection limit of the order of 0,3 ppm to 0,4 ppm, an alternative method is specified in annex A.