A method of treating biomass having fermentable material is provided utilizing acid hydrolysis in a countercurrent diffusion treatment structure. By practicing the invention acid usage is minimized, pentose concentration in the hydrolysate solution is maximized, and ethanol, butanol, butanediol, and the alcohols can be produced without the input of any external energy whatsoever into the production method. Biomass is particlized and slurried, and then is continuously subjected to acid hydrolysis at temperature, acid concentration, and residence time conditions sufficient to effect hydrolysis of the hemicellulose in the biomass to effect separation of pentose and hexose sugars into a hydrolysate having insufficient furfural to substantially inhibit fermentation microorganism growth, while not substantially hydrolyzing the cellulose in the biomass. Fermentation of the pentose and hexose sugars in the hydrolysate may then be effected, and alcohol produced from the fermented pentose and hexose sugars utilizing conventional techniques. Hydrolysis is practiced in an upright diffusion vessel with countercurrent flow, the acid concentration being about 2 to 10%, the temperature in the vessel being about 120.degree. C. or less, the residence time of the biomass in the vessel being about 1 to 3 hours, the biomass solids to liquid ratio being about 20/100 to 40/100 on a volume basis, and the particle size of the biomass being about 1-4 mm.