The effect of magnesium chloride (a Lewis acid) on the fractionation of aquatic plant cattail was studied using a Dionex accelerated solvent extractor varying salt concentration (0-0.4 M), treatment temperature (140-180 degrees C), and residence time (5-15 min). MgCl2 treatment enhanced xylan degradation and delignification, and up to 33.8% of total xylan was directly hydrolyzed to the xylose monomer. By removing xylan and lignin, the MgCl2 treatment effectively increased enzymatic digestibility of cattail cellulose and xylan. When cattails were treated with the 0.4 M MgCl2 at 180 degrees C for 15 min and subsequently hydrolyzed for 48 h with a cellulase loading of 15 FPU/g glucan, the highest glucose yield of 61.7% and the highest total xylose yield of 90.6% were reached, respectively. Application of Lewis acids as a novel biomass fractionation approach will require lower xylanase loading for hydrolysis, and have lower corrosion to equipment compared to other acid pretreatment processes.