A novel two-temperature dilute-acid prehydrolysis of hybrid poplar xylan that exploits the xylan biphasic kinetics at moderate temperatures is described. A lower temperature (140 degrees C) is applied to hydrolyze the easily hydrolyzable xylan, and a higher temperature (170 degrees C) is subsequently applied to hydrolyze the remaining xylan. Using a bench-scale percolation reactor, yields of soluble xylose expressed in monomeric xylose equivalents as high as 92% of theoretical have been achieved with only 2% of the xylan being degraded to furfural. The lignocellulosic substrate produced from the pretreatment is readily converted to ethanol at a yield of 94% of theoretical via a simultaneous saccharification and fermentation process in 48 h. In terms of both yield of xylose equivalents and ethanol production level and rate, these improvements are far superior to those previously reported using a single-temperature dilute-acid pretreatment.