Treatment with ammonia and hydrogen peroxide was investigated as a means of fractionating herbaceous biomass. The main feature of this process is that aqueous forms of these reagents are pumped simultaneously into a packed-bed flow-through-type reactor (percolation reactor) under a semibatch mode with ammonia being recycled. Experimental tests on corn cobs/stover mixture (CCSM) and switchgrass feedstocks have proven that a high degree of fractionation of biomass into three major components is attainable under this process scheme. The extent of delignification was 94-99% It was achieved at a representative condition of 170 degrees C, 0.28 g loading of H2O2/g biomass, and 10 wt% ammonia concentration. At the same time, about 80% of total hemicellulose in the biomass was separated out into the effluent primarily in the form of xylose oligomers. Decomposition of sugar components was insignificant. The remaining solids had a composition of 80-93% glucan, 5-10% xylan, and 1-6% lignin. Selected solid samples. obtained under near-optimum conditions, exhibited a chemical composition close tp that of commercial alpha-cellulose The enzymatic digestibilities of these solid samples were substantially higher than that of alpha-cellulose.