The lignin structure effect on the recalcitrance of biomass degradation was elucidated by twelve transgenic hybrid poplars (Populus nigra L. and Populus maximowiczii A.). Samples have different levels of the syringaldehyde (S) to vanillin (V) ratio from 0.1 to 2.6 and the lignin content from 10.5 to 24.3%, compared to the control (the S to V ratio 1.7 and the lignin content 22.0%). The dilute acid (0.1% sulfuric acid, 185 degrees C, and 30 min) and the green liquor (6% total titratable alkali (TTA), 25% sulfidity based on TTA, 185 degrees C and 15 min) were used as pretreatments. The enzymatic saccharification was performed at 5 FPU g(-1) based on the substrate. The removal of lignin and hemicelluloses varied with pretreatments. The high delignification originated from the high S to V ratio. However, the high S to V ratio retarded the enzymatic saccharification. The decrease of the lignin and the hemicelluloses benefited the enzymatic saccharification with or without the pretreatment. (C) 2013 Elsevier Ltd. All rights reserved.