Twelve transgenic hybrid poplars (Populus nigra L. and Populus maximowiczii A.) were used to demonstrate the influence of the lignin-carbohydrate complexes (LCCs) on enzymatic saccharification. The 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/V ratio 1.7 and the lignin content 22.0%). Any adventitious contaminants that could affect the final enzymatic saccharification were removed by performing thorough extraction on the samples. The crude milled wood lignins were subsequently isolated from which the phenyl glycoside, benzyl ether and gamma-ester linkages representative of the LCC were identified and quantified by C-13 and H-1-C-13 HSQC NMR. It was found that the samples showed different levels of the three LCC linkages, depending on the lignin content and/or the S/V ratio. The correlation between the LCCs and enzymatic saccharification nearly conclusively demonstrated that the LCCs accounted for the recalcitrance of lignocellulosic biodegradation. (C) 2013 Elsevier Ltd. All rights reserved.