Structural characterization of lignin extracted from the bio-oil produced by fast pyrolysis of switchgrass (Panicum virgatum) is reported. This is important for understanding the utility of lignin as a chemical feedstock in a pyrolysis-based biorefinery scheme. Pyrolysis induces a variety of structural changes to lignin in addition to reduction in molecular weight. The guaiacol structural units remain largely intact, and some hemicellulose stays covalently linked to the lignin. However, two-dimensional H-1-C-13 HSQC NMR analysis shows an absence of y-methylene hydrogens from beta-O-4 linkages, implying that rearrangements in the propyl linking chains have occurred. Peculate and hydroxyl phenol esters are still present in the pyrolyzed lignin, but at lower concentrations than in unpyrolyzed switchgrass lignin.