Because of its controllable and recoverable properties, the H2O-SO2 system was studied as a reaction medium to hydrolyze corn cob to produce chemicals by a two-step treatment. The results indicated that, after the first-step treatment of corn cob, 27.0% xylose, 3.3% glucose, and 3.2% arabinose could be obtained at 140 degrees C for 1.5 h with 0.024 g/cm(3) SO2; after the second-step treatment, 8.6% glucose could be obtained at 190 degrees C for 0.5 h with 0.048 g/cms SO2. The total yield of products by the two-step treatment of corn cob was 48.4%, which was greater than that obtained by the one-step treatment, 32.4% at 190 degrees C for 0.5 h with 0.048 g/cm(3) SO2. Furthermore, the degradation products obtained from the two-step treatment were less than those obtained from the one-step treatment. More importantly, after the reaction, SO2 could be swept by stream stripping and reused.