Selective liquefaction of lignin is important for synthesis of value-added phenolic monomers contributing to green chemistry and sustainable energy applications. In the present study, a synergistic effect of a supercritical carbon dioxide (scCO(2)) acidic catalyst in combination with a heterogeneous metal oxide catalyst, specifically nickel oxide (NiO) or ceria-doped scandia-stabilized zirconia (CeScSZ), in sub- or supercritical water (sbcr/scH(2)O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO(2)-assisted hydrothermal process in the temperature range of 100-400 degrees C resulted in highly selective synthesis of the phenolic monomers. On the basis of the total organic carbon (TOC) and gas chromatography mass spectroscopy (GC MS) analysis, it is evident that the scCO(2) catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO(2) and heterogeneous NiO catalysts resulted in the highest total relative yield of the phenolic monomers, reaching similar to 97% at 200 degrees C. It is confirmed that the scCO(2)-assisted hydrothermal depolymerization of the alkali lignin in the presence of both scCO(2) and a heterogeneous catalyst is superior to that of the each of these catalysts alone and in comparison of the systems where scCO(2) is replaced by inert nitrogen.