Dilute acid pretreatment processes for biomass hydrolysis have been studied for decades. These processes have been conducted in a variety of configurations such as low solids loading in batch, stirred tank reactors or in packed bed reactors operated in a “flow-through” or “percolation” mode that utilize a flowing dilute sulfuric acid liquid stream. Reported process yields from some of these configurations have achieved over 90% xylan hydrolysis with high-recovered yields of monomeric and oligomeric xylose.   To achieve cost-effective biomass hydrolysis using dilute-acid pretreatment a more likely process configuration would be continuous operation conducted at high solids loading, using appropriate pretreatment severities to achieve high levels of xylan hydrolysis while minimizing xylose degradation losses. However, continuous operation poses significant challenges in achieving uniform heat and mass transfer and in minimizing the distribution in residence time for the pretreated biomass solids. Variations in these factors can lead to both “under” and “over” pretreatment of the xylan fraction, leading to sugar losses and formation of degradation products. Using a statistical experimental design, combinations of temperature, sulfuric acid concentration, and rotational speed of the pretreatment reactor screw were used to identify conditions leading to high-monomeric xylose yields.  Using conditions identified from this designed set of experiments, a series of fifteen runs were carried out using a 200-kg/day high-solids, continuous, horizontal-screw pretreatment reactor.  Following each run, the solids were subject to a mild thermochemical secondary conversion step conducted at high solids loadings to convert more of the remaining xylo-oligomers to additional monomeric xylose. Results from the fifteen pretreatment experiments showed xylose yields of up to 80% and 85% for monomeric and total (monomeric + oligomeric) xylose, respectively. Furfural yields represented about 6% of the original xylan, with about 9% of the original xylan content remaining as unconverted xylan in the pretreated solids.