This International Standard specifies a method for the determination of the number of visible particles (including all kinds of contamination and black dirt) in cellulose acetate which are insoluble in a mixture of dimethylphthalate, dichloromethane and methanol, and are of size 0,15 mm or larger. This method is intended for cellulose acetate having an acetic acid yield above 50 % and containing no additives which affect the test results.