Empty fruit bunch (EFB) lignin, which had been isolated by the previous sulfuric acid treatment of EFB, was oxidized in the presence of a base and air to depolymerize the solvent‐insoluble lignin into smaller solvent‐soluble molecules. Depending on the types of base and solvent, the yield of solvent‐soluble lignin (SSL) varied, and the highest yield of 54% was obtained when MgO and ethanol were used at 150 °C with an air pressure of 500 psig for 3 h. Gel permeation chromatography analysis of the oxidized lignin showed that SSL had decreased molecular weight compared to native EFB lignin. 31P NMR experiments revealed that concentrations of hydroxy group and acid group were increased by the oxidation. 2D 1H‐13C HSQC NMR was performed, and it was found that the area intensities related to methoxy group and β‐O‐4′ unit as well as guaiacyl, oxidized syringyl, and p‐hydroxybenzoate group were decreased by the oxidation.