There is a commercial need to increase the amount of industrial produced lignin. An 86.24 wt.% formic acid solution with 0.2 M H(2)SO(4) as the catalyst was used to isolate the lignin of dewaxed wheat straw at different residence times and reaction temperatures. Lignin was extracted and purified separately from hydrolysates and residual solids after treatment. The lignin obtained has been characterized by FTIR (Fourier transform infrared), GPC (Gel permeation chromatography), and (1)H NMR (Nuclear magnetic resonance). Results from these analyses indicated that oxidative cleavage of bonds in the lignin macromolecule has occurred during the formic acid treatment, leading to formation of oligomers of lignin with lower molecular weight. Moreover, it was found that guaiacyl unit is the main structure, the content of syringyl unit is higher than that of the hydroxybenzyl unit and beta-O-4 together with beta-5 ether bonds is also identified in the molecular structure of lignin of wheat straw obtained from the formic acid process. (C) 2009 Elsevier Ltd. All rights reserved.