An ammonia-based pretreatment method termed ammonia recycled percolation (ARP) was developed for pretreating herbaceous biomass like corn cobs/stover mixture (CCSM) and switchgrass. The process involves treatment of biomass with aqueous ammonia through a percolation reactor (packed-bed, flow-through type). The effects temperature, reaction time, and ammonia concentration were studied. The extent of delignification in the ARP process was in the range of 60-80% for CSM and 65-85% for switchgrass. The ARP process solubilized significant amounts of the hemicellulose fraction into the pretreatment effluent, yet left most of the glucan fraction intact. The experimental data on CCSM and switchgrass indicate that the ARP is a highly effective pretreatment method. Near-complete conversion of cellulose to glucose was obtained by enzymatic hydrolysis of ARP-treated solid samples of CCSM, whereas conversion was slightly lower for switchgrass. The rate of enzymatic hydrolysis of ARP-treated samples was substantially higher than that of alpha-cellulose. The ARP effluents were evaluated for fermentability/toxicity by the xylose-fermenting yeast Pichia stipitis (NRRL Y-11545). The adaptability of ARP-treated solid samples to simultaneous saccharification and fermentation (SSF) was tested for ethanol production using cellulase enzyme and the yeast, Saccharomyces cerevisiae (NREL, D5A).