This part of ISO 13885 describes the determination of the molar-mass distribution, number-average molar mass Mn and mass-average molar mass Mw of polymers that are soluble in THF (tetrahydrofuran) by gel permeation chromatography (GPC)1). It is possible that, in spite of the good repeatability obtained with this method, it cannot be used with certain polymer types because of specific interactions, such as adsorption within the sample/eluent/column system. The method is not an absolute one and requires calibration with commercially available unbranched-polystyrene standards that have been characterized by absolute methods. The results for samples of polymers other than polystyrene are therefore only comparable within groups of samples of the same type. The conditions specified in this part of ISO 13885 are not suitable for the GPC analysis of polymer samples with Mw values greater than 106 (see Annex C). No correction methods, e.g. for the elimination of peak broadening, are included in this part of ISO 13885. If absolute molar-mass values are required, an absolute method, e.g. membrane osmometry for Mn or light scattering for Mw, must be used.