The fractionation behavior following steam explosion of three biomass resources, yellow poplar (Liriodendron tulipifera) wood chips, peanut hulls (Arachis hypogaea), and sugar cane (Saccharum officinarum) bagasse (three separate fractions, leaf, pith, and whole bagasse) were examined following steam explosion using a Stake II reactor. Component separation was evaluated using a range of different severity factors. Water solubles, alkali solubles, and an insoluble Fraction were collected separately and subjected to component analysis. The latter addressed both chemical (component analysis by NMR and UV spectroscopy) and molecular structure (molecular weight determinations). The water soluble fraction consisted primarily of xylose (mono and oligo-saccharides) and water soluble lignin; the alkali soluble fraction contained most of the lignin and unidentified alkali-soluble polysaccharides; and the residual fiber was mostly cellulose. Whereas the hemi-celluloses could not be preserved as polymers, lignin had molecular weights (M-n) in excess of 3000 and dispersities in excess of 10, except for highly severe reaction conditions which produced lignins with lower molecular weight. The molecular weight of the cellulose declined steadily with reaction severity; that of lignin dropped abruptly at a severity of log R-0 4.25 where homolytic depolymerization was indicated. Fraction behavior and fraction character were found to be highly dependent on the severity of the steam explosion treatment.