SO2-catalyzed steam explosion effectively fractionates softwood carbohydrates, releasing 80-90% of the hemicellulose into solution and enhancing the enzymatic hydrolysis of the water-insoluble cellulose fraction. When steam pretreatment conditions were modified to ensure maximum hemicellulose recovery, a significant Dart of hemicellulose-derived carbohydrates were solubilized in an oligomeric form that was not readily fermented. When Douglas fir wood chips were used as the substrate, sulfuric acid-catalyzed post-hydrolysis at 120 degrees C for 1 h allowed most of the original hemicellulose to be recovered in the monomeric form. Although higher catalyst concentrations (1-3%) resulted in complete depolymerization of the carbohydrates, noticeable losses due to sugar degradation were observed. Lower concentrations of the added catalyst increased the monomer yields, and resulted in less secondary degradation. Partial oxidation of the added SO2 to sulfuric acid during steam explosion resulted in the depolymerization of the hemicelluloses during post-hydrolysis, even without further acid addition. Hexoses present in the post-hydrolyzates could be readily fermented to ethanol.