CO sub(2)-pressurized liquid hot water was applied to pretreat Sorghum stalk to increase its accessibility for enzyme hydrolysis. Pretreatment temperatures tested ranged from 180 to 220 degree C, and reaction times varied between 10 and 30 min. The highest total reducing sugar yield of 81.54% was obtained for the sample pretreated at 200 degree C and 20 min. Different from the N sub(2)-pressurized liquid hot water process, the generated carbonic acid in the CO sub(2)-pressurized method acted as catalyst in the pretreatment process. It was found that the reducing sugar yield increased significantly at lower temperature and shorter reaction time with the addition of CO sub(2). Finally, X-ray diffraction and scanning electron microscope analysis were used to investigate the microstructure of the Sorghum stalk before and after the pretreatment. copyright 2016 American Institute of Chemical Engineers Environ Prog, 36: 208-213, 2017