During biomass depolymerization to monosaccharides, a pretreatment step is crucial for allowing cellulose accessibility to hydrolytic enzymes. However, this step can have high-energy demands and significant chemical inputs, and can introduce dilution into the whole process. These issues will negatively affect process sustainability and it is crucial to develop technologies that minimize their impact. A high pressure (200 bar) CO2-H2O pretreatment process was developed with these issues in mind. Hardwood was pretreated at 20 and 40 wt% solids (for the H2O- biomass solution), while switchgrass and mixed perennial grasses (a co-culture of big bluestem and switchgrass) were pretreated at 40 wt% solids at temperatures ranging from 150°C to 250°C, and residence times between 20 s to 60 min. All samples were hydrolyzed using 15 FPU/(g cellulose) of Spezyme CP® cellulases, and 30mg/(g cellulose) of Multifect® xylanase. After those two steps total yields were determined for glucose, hemicellulose sugars, furfural and 5-HMF. Surface-response curves of yield vs temperature and residence time were generated and compared between the different moisture contents and biomass types. Pretreatment at 170°C for 60 min gave glucan to glucose yields of 73% for both 20 and 40wt% solids mixed hardwood, while it led to 79% for 40wt% solids Switchgrass. Pretreatment at 160°C for 60 min gave glucan to glucose yields of 67% and 81% yields for 40% solids mixed hardwood and switchgrass, respectively.