Scale-up of the aqueous ammonia soaking (AAS) biomass pretreatment method to 75-L soaking vessel size was accomplished in this work. A novel, pilot-scale AAS system capable of pretreating 4 kg of switchgrass per cycle was designed, fabricated, and tested. The pretreatment process involved soaking biomass in 29.5% aqueous ammonium hydroxide at a liquid: solid ratio of 5 L/kg. Major vessel design criteria included (1) allowing thorough washing of the soaked biomass in the pretreatment reactor; (2) simple, low-cost fabrication; and (3) limiting safety hazards by minimizing ammonia fumes from the system. Based on these constraints, commercially available 75-L HDPE tanks were selected as the primary vessels for pretreatment. The pretreatments were conducted outside, without agitation during the summer months in Iowa, with ambient temperatures ranging from 15°C to 33°C. During the first experimental cycle, clogging of the outlet resulted in leakage from the vessel during rinsing, and led to redesign of the washout prevention system. The redesigned system used a "teabag" approach in which dry biomass was preloaded into a cylindrical mesh bag, and the filled bag was placed into the soaking vessel. This modification eliminated outlet clogging, simplified biomass loading and unloading, but slightly reduced washing efficiency. Through five soaking cycles, an average of 22% to 25% delignification was achieved (Klason lignin basis) compared to the 35% removal seen at the bench-scale as reported by our group. Approximately 50% to 60% of the pretreated switchgrass was recovered, dry basis, compared to 75% previously achieved at the bench-scale. Overall, the system successfully generated moderate quantities (10 kg/wk) of pretreated biomass for pilot-scale fermentation experiments while illustrating some of the materials handling challenges that must be addressed as pretreatment methods are scaled-up.