Abstract: A method for producing a fermentable wort useful to produce ethanol from cellulosic materials is particularly adaptable for batch-wise processing. The broad method includes the steps of placing the materials in an airtight vessel that is evacuated to a moderate vacuum at a reduced temperature of 20.degree. C. or less. Highly concentrated mineral acid gas, such as HCl, is introduced to raise the vessel pressure to a pressure less than 1500 mm of Hg wherein the concentration of acid gas is at least 40%. These temperatures and pressures are maintained for at least 30 minutes to depolymerize the cellulose after which the vessel is again evacuated to remove the excess gas and then raised to ambient pressure so that the material is removed. The depolymerized material, which contains some adsorbed acid gas, is mechanically conveyed against a counterflow of hot water.