High-molecular weight lignins (HML) and low-molecular weight lignins (LML) were extracted from flax shives using pressurized 30% aqueous ethanol, and isolated by precipitation and ultrafiltration. The isolated lignin samples and two commercial lignins were assayed for monolignol content and functional groups, solubility assays, thermal analysis using differential scanning calorimetry (DSC), size exclusion chromatography (SEC), and Fourier transform infrared spectroscopy (FT-IR). The highest yield of HML was 13.5 g/100 g of flax shives, and this was about six times higher than the LML yield. SEC of lignin samples revealed that the molecular weights of HML and LML are about 30,000 and 8749 Da, respectively. The DSC results of HML show that this lignin has a glass transition temperature near 160 degrees C. HML contains fewer carboxyl groups and more phenolic hydroxyl groups than LML. The higher molecular weight and higher thermal stability of HML makes it an excellent candidate for industrial applications. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.