The molecular structures of the milled wood lignins (MWLs) and technical lignins (TLs) obtained from four hardwood species (Eucalyptus urograndis, Eucalyptus nitens, Eucalyptus globulus, and Populus trichocarpa) were quantified by C-13 nuclear magnetic resonance (NMR) in tandem with their thermal responses, as obtained by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both MWLs and TLs showed similar DSC curves with two exothermic peaks (referred to as peaks 1 and 2). Also, maximum temperatures for MWL peaks were higher than for TL peaks, with an average of 20 and 10 degrees C for the first peak and second peak, respectively. Even though calculated enthalpies for MWLs were higher because of their purities, TLs had reasonable relationships between heat values and lignin substructures. TL had a positive correlation between condensed structures and the enthalpy value between 413 and 428 degrees C, but negative relationships were found at the latter temperature window for lignin substructures, such as methoxyl groups, syringyl/guaiacyl ratio, and aliphatic OHs.