Flowing compressed hot water through cellulosic biomass enhances removal of total mass, xylan, and lignin and increases cellulose digestibility compared to batch operations at otherwise identical conditions. Staged countercurrent operation with very dilute sulfuric acid has been shown to provide similar benefits. This study focused on developing comprehensive information on the combined effects of flow and very low concentrations of sulfuric acid (0.05-0.1 wt %) on hemicellulose hydrolysis in corn stover at 180 degreesC. A flow rate of 10 mL/min in a 3.8-mL reactor enhanced xylan removal by about 25% compared to batch operation at the same conditions. More than half of the dissolved xylan was oligomeric, and the oligomer fraction increased with flow rate but decreased with acid concentration. Increasing fluid velocity at a constant residence time also significantly accelerated xylan solubilization. Although lignin removal was limited to about 12% without flow, up to half of the lignin was swept out with flow, and lignin removal could be related to hemicellulose dissolution at high flow rates. Enhancement of xylan removal at higher flow rates could be attributed to replenishment of sulfuric acid to compensate for neutralization by biomass minerals, but this possibility does not account for similar observations without acid addition. Thus, it is postulated that both mass transfer and chemical reaction influence hemicellulose solubilization. Furthermore, it appears that hemicellulose oligomer-lignin compounds dissolve initially but release lignin fractions that are less soluble or form low-solubility condensation compounds when held for extended times.