The cooking with solid alkali and active oxygen has a high selectivity for delignification. In the present work, the O-2 and H2O2 were separately combined with MgO used in cornstalk cooking for investigating their effects on delignification. After cooking, the lignins in raw material, pulp, and yellow liquor were all characterized by HSQC NMR. The results showed that the syringyl (S/S'/S '') units and beta-O-4' (A/A'/A '') structures had different reactivity in the cooking with MgO and H2O2 due to their different structures on side-chains. Whereas the syringyl (S/S'/S '') units could be completely decomposed when the MgO and O-2 were used, and the beta-O-4' (A/A'/A '') structures could be partly degraded. A novel structure G' unit with a carbonyl group was only generated in the cooking with MgO and O-2. In addition, the H unit, non-phenolic beta-beta' (B) and beta-5' (C) structures were all stable in both of the two cooking processes. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.