TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method (M1), after step (B) the vessel is evacuated to a partial vacuum. After the step (D) the pretreated biomass is recovered. At the step (C) the anhydrous ammonia is in contact with the biomass for at least 2 minutes. The anhydrous ammonia is in contact with the biomass for less 24 hours. The temperature of the biomass in the vessel is at least 70 degrees C. The temperature of the vessel is raised to at least 70 degrees C prior to step (B). The temperature prior to step (B) is 70-190 degrees C. The temperature is raised directly by injecting hot gas or indirectly by application of heat to the vessel. The ammonia is recovered at step (D) and recycled to a pretreatment vessel or to an ammonia vapor handling system. The ammonia is recycled in an anhydrous state. The pressure in the vessel is 0.1 atmosphere absolute to less than 20 atmospheres gauge over steps (B) through (D). The biomass of (A) is mechanically size-reduced biomass. Preferred Components: The concentration of anhydrous ammonia at step (C) is less than 20 wt.% measured as a function of dry weight of the biomass in the vessel. EXAMPLE - Corn cob (4.07 kg) was and classified to obtain a cob fraction with particle diameter greater than 1/4 inch (0.64 cm) and less than 3/8 inch (0.375 cm). The cob was placed into a basket which was then placed into the pretreatment reactor. The corn cob had a moisture content of 8.6% and a bulk density of 227.5 g/l. The pre-reaction, packed bed depth measured approximately 29 inches (73.7 cm). Air was evacuated from the reactor and a partial vacuum of 0.1 bar (10 kpascal) was achieved. Anhydrous ammonia (8 wt.%) based on mass of bone-dry biomass, was added by charging ammonia (297.6 g) for approximately 60 minutes. Saturated steam at a pressure of 84.7 psia (584 kpascal) was then added to bring the reactor temperature to approximately 70 degrees C. At this time, the temperature of the reactor was allowed to increase from 70-90 degrees C over a hold time of 40 minutes. The pressure in the reactor was approximately 85 psia (584 kpascal) during the hold time. After the hold time the pressurized gas and vapors were released and captured in a cooled, external-containment vessel. When the reactor reached atmospheric pressure, the reactor was then evacuated to approximately 0.1 bar (10 kpascal) using a vacuum pump, and held at that pressure for five minutes. The reactor was then vented to relieve the vacuum to atmospheric pressure. The pretreatment reactor was opened after the process reached atmospheric pressure. The pretreated biomass was removed in quartile fractions from the top to the bottom of the packed-bed, i.e. first (top) quarter, second quarter, third quarter, fourth (bottom) quarter. These fractions were analyzed for moisture content and residual ammonia and results showed that moisture contents for first, second, third and fourth quarters were 21.42, 5.82, 6.07 and 28.78 wt.% respectively; and concentration of NH3 for first, second, third and fourth quarters were 0.275, 0.122, 0.133, and 0.331 g/100g dry matter respectively. Residual ammonia was determined by extracting a known amount of pretreated solids in water for one hour, then using titration with 0.1N HCl to determine the equivalents of acid to reach pH 5.3. The equivalents were then normalized to the amount of dry matter in the extraction system. In addition, equal masses of each fraction were subsequently combined, then analyzed, to determine a mixing-cup composition of the pretreated biomass.