In this work, the structural features of the lignin isolated with 2% NaOH at 90 degrees C for 2.5 h from the hydrothermally pretreated eucalyptus fibers at different temperatures (100-200 degrees C) for different times (1560 min) were thoroughly investigated. Results showed that the hydrothermal pretreatment facilitated the separation of alkali lignin from the pretreated fibers. It was found that the linkages of beta-O-4, beta-beta, and beta-5 decreased gradually with the increase of hydrothermal severity. Furthermore, decreased molecular weights (1630-510 g/mol), associated carbohydrates contents (1.99-0.05%) and aliphatic OH contents (3.37-0.65 mmol/g), and increased phenolic OH contents (0.71-2.98 mmol/g) and thermal stability of the alkali lignins were observed with the increase of the hydrothermal severity. (C) 2014 Elsevier Ltd. All rights reserved.