Lignin was hydrothermal oxidation depolymerized by using H2O2 as the oxidant and CuO/Fe-2(SO4)(3)/NaOH as catalysts. The effect of the solvents, including water, methanol, 1,4-dioxane, tetrahydrofuran, ethanol, and their co solvent, on the depolymerization of lignin were investigated. It is found that the yield of monophenolic compounds reaches 17.92% by using 1:1 (v/v) of methanol/water as the reaction medium. The suitable proportion of methanol to water not only effectively maintains the alkalinity of the reaction medium and the solubility of lignin, but also suppresses the demethoxylation during the process of depolymerization reaction, resulting in an increase of contents of syringyl style monophenolic compounds which prevents the re-polymerization. Consequently, the yield of monophenolic compounds is increased. (C) 2015 Elsevier B.V. All rights reserved.