Provided are methods for the solubilization of cellulose into soluble sugars without the need for high temperatures, high pressures, strong acid solutions, and/or added water. The produced sugars can be fermented into ethanol. In one embodiment, the method comprises contacting a cellulose-containing material with a solid acid material and agitating the cellulose-containing material and the solid acid material for a time sufficient to produce an aqueous solution comprising a quantity of soluble sugars.