Lignins impart rigidity to woody plants, achieved by cross-linking different plant polysaccharides that strengthen the cell walls, and are the second most abundant organic polymers on earth. The name itself is derived from the Latin word lignum, meaning wood. Lignins are important in the papermaking process – it is lignin in the pulps used to make newspapers which causes them to yellow with age and it must be removed to produce high quality bleached paper. The base monomers are phenylpropanoids, aromatics with carboxylic acid and hydroxyl functionalities, which form highly complex, amorphous, three-dimensional polymers. It is also used as a raw material for several chemicals, often via lignosulfonates. Determining the molecular weight distribution and structural properties of lignins is an important step in understanding their behavior in wood pulps. However, the analysis is very challenging, as they are very difficult to extract from wood pulps and often degrade during this process. There is a huge variation in the structure of lignins, with the material showing great heterogeneity from one source to the next. The accurate molecular weight distributions of four samples of lignin were investigated by GPC with viscometry.