Pretreatment of biomass substrates is critical in their use for generating monomeric sugars for conversion to transport fuels, such as bioethanol. In this study, high pressure reactive pretreatment and hydrolysis of cellulose- and hemicellulose-containing substrates, principally corn stover and switchgrass, were conducted over the temperature range of 150-190 degrees C utilizing carbon dioxide pressures ranging from 150 to 450 bar. Experimental protocol was guided by the development of an orthogonal design criteria consisting of temperature, CO2 pressure, time of pretreatment-hydrolysis, and substrate particle size. High pressure carbonation of the subcritical water-biomass mixtures were conducted in a small batch reactor and the resultant hydrolyzate mixtures were analyzed for sugar content using SEC and HPLC-RI detection. Also, the resultant hydrolyzates after carbonated water pretreatment were further hydrolyzed using commercial enzymes to saccharify the remaining oligomeric sugars to xylose and glucose. The resultant % sugar content of the carbonated water - versus dilute mineral acid - pretreated biomass mixtures were compared as well as the corresponding final enzymatically saccharified hydrolyzates for switchgrass and corn stover. High pressure carbonated water pretreatment yielded 9-13% less sugars than the sulfuric acid-derived hydrolyzate and relatively 6-10% less sugars were found in the final carbonated water treated hydrolyzates after further enzymatic treatment. It was also found that carbonated water pretreated switchgrass hydrolyzate required 33% lesser amount of enzyme for saccharification when compared to that obtained using dilute sulfuric acid pretreatment. The supercritical carbon dioxide dissolved in water provides an environmentally benign and "green" pretreatment method for depolymerization of the sugars present in biomass substrates and to facilitate further saccharification without the necessity of base neutralization, or pH adjustment prior to application of enzyme-initiated hydrolysis. (c) 2012 Elsevier B.V. All rights reserved.