Absolute molecular weights of several commercially and semi-commercially available lignins were determined by gel permeation chromatography (GPC with a differential viscosity detector (DV). Solubility in THF was assured by acetylation. Polystyrene molecular weight standards were used for establishing a universal calibration curve. The lignins included those from hardwood, softwood, and sugar cane bagasse; and these were isolated by the kraft or organosolv pulping process, or by steam explosion/autohydrolysis. All lignins exhibited more or less uniform distributions with weight average molecular weights (Mw) between 3, 000 and 20, 000; with polydispersities (Mw/Mn)between 2 and 12; with Mark-Houwink-Sakurada exponential factors (α) between 0.17 and 0.35; and with intrinsic viscosities between 0.037 and 0.08 dLg−1. A significant relationship between Mw/Mnand Mwwas discovered that had a correlation factor of 0.92. This relationship has the form of Mw/Mn= 0.45 (Mw10−3+ 0.85.