Polysaccharides such as agar, guar gum, starch, and xylan were hydrolyzed to produce mono- and oligosaccharides under hydrothermal conditions with and without carbon dioxide in a small batch reactor. The molecular weight distributions of the polysaccharide hydrolyzates shifted to lower molecular weights by increasing the carbon dioxide load, corresponding to higher pressures of carbon dioxide. For example, the yield of glucose produced from the hydrolysis of starch at 200 degrees C was increased significantly from 3.7% to 53.0% (on a carbon weight basis) of the initial polysaccharide by increasing carbon dioxide load in a reaction time of 15 min. Carbonic acid generated from water and carbon dioxide appeared to lower the pH of high-temperature and high-pressure water. Polysaccharide hydrolysis under hydrothermal conditions in the presence of carbon dioxide is an environmentally benign method to produce mono- and oligosaccharides because the process does not require the use of conventional acids and bases followed by neutralization and separation.