NOVELTY - Treating (M1) biomass involves in a reactor, allowing gaseous ammonia to condense on the biomass and react with water present in the biomass to produce pretreated biomass, where reactivity of polysaccharides in the biomass is increased during subsequent biological conversion as compared to polysaccharides in biomass which has not been pretreated.    USE - For treating biomass useful in system for producing biofuel or biochemical (claimed).    ADVANTAGE - The method provides biomass containing polysaccharides that has increased reactivity during subsequent biological conversion as compared to polysaccharides in biomass which has not been pretreated; and is efficient and economical.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) treating (M2) biomass involving in a reactor, impregnating biomass with an amount of ammonia to initiate a reaction, where the biomass contains water; and allowing a gaseous carrier delivered to the reactor at an elevated temperature to remove residual ammonia in the biomass to produce pretreated biomass free from ammonia, where the amount of ammonia is reduced as compared to reacting the ammonia and water without the gaseous carrier;    (2) treating (M3) biomass involving in a reactor, allowing ammonia to contact the biomass and react with water present in the biomass to produce pretreated biomass, where reactivity of polysaccharides in the biomass is increased during subsequent biological conversion as compared to polysaccharides in biomass which has not been pretreated; and    (3) a system comprising a biofuel or biochemical production facility; and a reactor at an elevated temperature containing biomass and ammonia, where the biomass contains polysaccharides, where the reactor is adapted to allow the ammonia to contact the biomass and react with water present in the biomass to increase reactivity of the polysaccharides during subsequent biological conversion as compared to polysaccharides in biomass which has not been pretreated.