Supercritical water gasification is an innovative handling method of black liquor, which can also produce hydrogen-rich gases. In this study, the catalytic activity of 14 common metal oxides on SCWG of black liquor were investigated and the catalytic mechanism was studied through X-ray photoelectron spectroscopy (XPS) analysis. All the tested oxides improved the gasification efficiency and V2O5, WO3 and Co2O3 showed the highest performance. The H-2 yields of 21.67 and 21.03 mol/kg were achieved with the presence of Co2O3 and ZnO respectively. The gasification efficiency increased with the increasing V2O5 amount, but the H-2 fraction increased firstly and then decreased. The highest H-2 fraction of 44.59% was obtained with V2O5 loading amount of 45 wt%. The increase of reaction time improved the CE, but the further prolongation had little influence. The XPS analysis of CuO and Fe2O3 showed some of them were reduced to metal (Cu) or lower-valence oxides (Cu2O, FeO). The metal oxides probably promoted black liquor decomposition by introducing reactive oxygen, and the generated metal also catalyzed the gasification. Accordingly, an oxidation reactor could be introduced to recycle the metal oxides, which may help to realize the complete gasification of black liquor or other refractory wastes at milder temperatures. (C) 2020 Elsevier Ltd. All rights reserved.