A recently installed 100-L horizontal shaft custom-fabricated mixer/reactor, made of Carpenter 20 Cb-3 stainless steel and designed for high-solids, dilute-acid pretreatment of lignocellulosic biomass, is described. Several preliminary runs with a hardwood and a herbaceous feedstock are reported; the results show the operation to be reliable with good reproducibility. No significant differences between comparable pretreatments in the high-solids reactor and a 1-L laboratory-scale reactor are apparent. Pretreatment efficiency is not affected by 10-15% (w/w) solids loading or biomass particle size of up to -10 mm. Mass balances are closed reasonably well, with improvement in reactor contents handling and biomass analytical methods desirable.