This part of ISO 1628 defines particular conditions for determining the reduced viscosity (also known as viscosity number) and intrinsic viscosity of polyethylenes and polypropylenes at 135 °C in dilute solution.

The viscosity of polymer solutions may be affected by additives present in the sample. The value of a reduced viscosity determined by this method may therefore be unreliable if the sample contains fillers or other additives.