This method is suitable for cellulose acetate which does not contain additives, fillers, etc. which may interfere with the determination of viscosity. It is suitable for cellulose acetate having an acetic acid yield above 50 %. Cellulose acetate plasticized with dimethyl phthalate is moulded under specified conditions of temperature, pressure and time. After cooling, the moulding is ground up. The viscosity ratio of the ground material from the moulding and also that of the original cellulose acetate material are determined in accordance with ISO 1157. The percentage viscosity loss is calculated from the values of viscosity before and after moulding. This second edition cancels and replaces the first edition ISO 1599:1975.