The molecular weight distributions (MWDs) of dissolved and residual lignins from different delignification phases during flow-through kraft cooking of birch have been studied. The fundamental characteristics of the isolated lignin samples were evaluated and the lignins were characterised by size exclusion chromatography using lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) not used for isolated lignin samples before. The MWDs of underivatized and acetylated samples were compared with the MWDs of acetylated samples obtained by using tetrahydrofuran (THF) as mobile phase in a similar chromatographic system.