Currently, sub-/supercritical fluids are becoming one of the most popular sustainable technologies for biomass processing since they have demonstrated several advantages over conventional processes. Enhanced selectivity, better hydrolysis capabilities and lower environmental impacts are some of the crucial aspects in the important growth of sub-/supercritical fluids field. This chapter focuses on the direct application of the most natural solvents, CO2 and H2O in a mixture and at high pressures, in hydrolysis of biomass, taking cellulose, hemicelluloses, lignin, starch and proteins as representative polymers for the direct production of output chemicals. Fundamentals of high-pressure CO2 and a CO2/H2O mixture are reviewed, together with the essential parameters that can affect the efficiency of biomass hydrolysis and the tuning of them. Although the application of high-pressure CO2/H2O faces several drawbacks concerning the equipment costs and scale-up, this novel technology holds great promise for utilization as a "green" technology in a biorefinery setting.