Pretreatment experiments were carried out with the goal of achieving high xylose yields in current high solids pretreatment batch reactors to serve as a benchmark for further increases in xylose conversion.  Corn stover was pretreated with dilute sulfuric acid using a four liter Steam Gun and a two liter stirred ZipperClave(®) reactor at an initial solids loading of 45% and with nominal particle sizes of either 6 or 18 millimeters.  Acid impregnation was performed prior to pretreatment by soaking the biomass in an acid bath at 60^o C for four hours and then dewatering the biomass to 45% total solids in a hydraulic press. Pretreatment was carried out at temperatures between 180°C and 200 °C for residence times of either 90 or 105 seconds.  Results demonstrate an ability to achieve high xylose yields over a range of pretreatment conditions, with performance showing little dependence on particle size or pretreatment reactor type.  The highest yield of xylan to total soluble xylose (monomer and oligomer) was 87% at 180°C for 90 seconds using the steam gun, with 82% recovered as monomers.  The total xylan removal was 93% under these conditions, with a xylan loss to furfural of 4%.   High xylose yields reported in this study were attributed to effective catalyst impregnation, along with operating at high solids loadings that permit effective steam penetration.  The pretreatment results obtained are among the highest published for dilute acid pretreatment and high solids concentrations, and provide a benchmark for achieving higher xylan-to-xylose yields in continuous reactors.