This International Standard specifies a method for the determination of the viscosity number (also referred to as reduced viscosity) of dilute solutions of polyamides in certain specified solvents. The method is applicable to the polyamides designated PA 46, PA 6, PA 66, PA 69, PA 610, PA 612, PA 11, PA 12, PA 6T/66 and PA MXD6 as defined in ISO 1874-1, as well as to copolyamides and other polyamides that are soluble in one of the specified solvents under the specified conditions. The method is not applicable to polyamides produced by anionic polymerization of lactams or produced with crosslinking agents; such polyamides are normally insoluble in the specified solvents. In general, polyamide samples have to be completely soluble in the solvents mentioned, and the additives contained in them (e.g. reinforcement fibres, flame-retardants and modifiers) must not interfere with the viscosity measurement. However, the method can be used for production control of compounds containing additives which interfere with the viscosity measurement. In such cases, a particular compound in a particular solvent will have a specific viscosity number which, in principle, cannot be converted from one solvent to another except by means of a relationship specific to the compound concerned. Viscosity numbers converted in this way are only suitable for inter-product comparisons. The viscosity number is determined by the general procedure specified in ISO 1628-1, observing the particular conditions specified in this International Standard. The determination of the viscosity number of a polyamide provides a measure of the relative molecular mass of the polymer.