Sustainably producing concentrated solutions of monosaccharides from biomass is a key challenge facing the conversion of lignocellulosic biomass to biofuels or bioproducts. Most pretreatment and enzymatic hydrolysis processes are run at low-solid concentration (<10 wt%) and use chemical catalysts, while most high-solids enzymatic hydrolysis experiments are performed with air-dried pretreated materials. Using optimally two-temperature stage CO₂–H₂O pretreated biomass substrates (210 °C 16 min, 160 °C, 60 min for mixed hardwood and 210 °C, 1 min, 160 °C, 60 min), high-solids enzymatic hydrolysis reactions were performed in a novel high-solids reaction system. With this system, twelve “rotating drum” reactors were run simultaneously in a controlled environment. Without additional chemical catalysts or any drying, two-temperature stage CO₂–H₂O pretreatment coupled with high-solids enzymatic hydrolysis produced monosaccharide solutions of 185 g L⁻¹ for mixed hardwood and 149 g L⁻¹ for switchgrass. Apart from results obtained with dilute acid pretreated corn stover, these are the most concentrated solutions obtained from biomass pretreatment and enzymatic hydrolysis without substrate drying. The corresponding glucan to glucose yields were above 80% for both types of biomass. Notably, these high yields were obtained because, similar to dilute acid pretreatment but unlike un-catalyzed pretreatment, our approach produced biomass that did not show decreasing yields with increasing enzymatic hydrolysis solid contents. These results suggest that CO₂–H₂O pretreatment is an attractive alternative to chemically catalyzed processes such as dilute acid pretreatment.