NOVELTY - Pretreating lignocellulosic biomass comprises mixing the lignocellulosic biomass with either a sulfite solution or a hydroxide solution and maintaining the mixture for a first treatment period; and subsequently decreasing the pH of the mixture by introducing a pH-reducing agent into the mixture and maintaining the mixture for a second treatment period.    USE - The methods are used for pretreating lignocellulosic biomass; and producing ethanol from lignocellulosic biomass (all claimed).    ADVANTAGE - Low cost methods of pretreating lignocellulosic biomass in preparation for enzymatic saccharification and fermentation are provided.    DETAILED DESCRIPTION - Pretreating lignocellulosic biomass comprises mixing the lignocellulosic biomass with either a sulfite solution having an initial pH of greater than or equal to 3 or a hydroxide solution having an initial pH of greater than or equal to 9, as measured at room temperature, and maintaining the mixture at 150-200 degrees C for a first treatment period; and subsequently decreasing the pH of the mixture to a pH of 1-3 by introducing a pH-reducing agent into the mixture and maintaining the mixture at 150-200 degrees C for a second treatment period to provide a treated product comprising pretreated biomass solids and a process liquor. An INDEPENDENT CLAIM is a method of producing ethanol from lignocellulosic biomass by mixing the lignocellulosic biomass with either a sulfite solution having an initial pH of greater than or equal to 3 or a hydroxide solution having an initial pH of greater than or equal to 9, as measured at room temperature, and maintaining the mixture at 150-200 degrees C for a first treatment period; subsequently decreasing the pH of the mixture to a pH of 1-3 by introducing a pH-reducing agent into the mixture and maintaining the mixture at 150-200 degrees C for a second treatment period to provide a treated product comprising pretreated biomass solids and a process liquor; and subjecting the pretreated product to enzymatic saccharification and fermentation to produce ethanol.