Method for determining the viscosity number of diluted polyamide solutions in certain specified solvents. The polyamide samples must be completely soluble in the specified solvents. The method is applicable to polyamides designated PA 46, PA 6, PA 66, PA 69, PA 610, PA 612, PA 11, PA 12, PA 6T/66, PA 6I/6T, PA 6T/6I/66, PA 6T/6I, PA 6I/6T/66 and PA MXD6, as defined in ISO 1847-1, and copolyamides, polyamide compounds and other polyamides which are soluble in one of the specified solvents under the specified conditions. The method is not applicable to polyamides prepared by anionic polymerisation of lactams or with cross-linking agents; such polyamides are normally insoluble in the specified solvents. The viscosity number is determined according to the general method laid down in ISO 1628-1, subject to the special conditions laid down in this document.