RISE Bioeconomy, FPInnovations and VTT Technical Research Centre of Finland Ltd are working on a draft for an official ISO method for the determination of lignin molar mass. The draft will include two methods that can be used by different laboratories as reference methods, which are based on organic and aqueous size exclusion chromatography (SEC). Four samples of kraft lignin of different origin were analysed by three laboratories using the current in-house methods as well as a harmonised analytical setup. It was clear that the results differed largely between the different methods, for example, the weight average molar mass (Mw) for the LignoForce softwood lignin (LF-SW) ranged from 3,300 t0 11,100g/mol using the different methods. Harmonisation of aqueous SEC was successful. The method comprises SEC using 0l.1M sodium hydroxide as the mobile phase, a mobile phase flow rate of 0.5mL/min, Waters columns MCX 1,000A and 100,000A in series and calibration/calculation using polystyrene sulphonate standards using a linear fit. However, organic SEC was more complex and would require further investigations. Although the aqueous SEC analysis was repeatable, it was concluded that a strategy for data evaluation should be developed in order to achieve a method applicable for quality assessments.