Converting biomass to useful chemicals through economically viable processes is crucial for sustainable development. Subcritical CO2 and supercritical CO2 are nontoxic, nonflammable, and inexpensive reagents, and the H2O/CO2 mixture is strongly acidic, facilitating biomass hydrolysis. This study investigated the catalytic activity of the H2O/CO2 system under high pressure. We studied the decomposition behavior of bamboo under hydrothermal and hydrolysis conditions with H2O/CO2 in a semicontinuous-flow reactor at 9.8 MPa of pressure. At a severity factor of 5.2, the solid yield of hydrolysis with CO 2 was significantly lower compared with that of the hydrothermal reaction (7.3 vs 25.8 wt %, respectively). Hydrolysis with CO2 under severe conditions also exhibited effective degradation of glucan and xylan. The results showed that a linear relationship exists between the catalytic activity of the H2O/CO2 system and the severity factor. Also, the severity factor is closely related to the reaction temperature under our experimental conditions.