Composition of biomass is one of the key factors affecting the pretreatment. To further delineate it, we have applied pretreatments based on aqueous ammonia to three different substrates with widely varying compositions, i.e., corn stover, high lignin hybrid poplar, and low lignin hybrid poplar. Pretreatments were applied by two different methods: Ammonia Recycle Percolation (ARP) and Soaking in Aqueous Ammonia (SAA). ARP and SAA are both highly efficient in delignifying biomass. Yet they retain most of the carbohydrates in the solid after pretreatment. Since most cellulases have xylanase activity as well as cellulase activity, sugars are obtained directly from solid by enzymatic hydrolysis without solubilizing hemicellulose sugars to liquid. This is a significant economic benefit since it eliminates the need of detoxification of hemicellulose sugars. For treated corn stover, the overall enzymatic digestibility was in the vicinity of 90% with 15 FPU loading of Spezyme CP. With poplar feedstocks, the digestibilities were lower than that of corn stover. In order to achieve acceptable level of enzymatic digestibility of both glucan and xylan, supplementation of external xylanase was necessary. Xylanase supplementation increases not only the digestibility of xylan but also the digestibility of glucan. With xylanase addition, the overall sugar yield from SAA/ARP treated low lignin hybrid poplar has increased from 70%/61% to 90%. Although the digestibilities of ARP or SAA treated corn stover is higher than poplar in general, the net increase of digestibility by xylanase addition was insignificant.