Gasification of lignin biomass model compounds was examined in the presence of magnesium oxide supported nickel catalysts (Ni/MgO) in sub-and supercritical water from 523 to 673 K. The main gas products were methane, carbon dioxide, and hydrogen. The amount of gases produced increased with an increase in nickel loading on magnesium oxide. The highest total gas yield in a carbon basis was 78% with 20 wt % Ni/MgO catalyst at 673 K and 0.3 g/cm(3) water density. In this system, the metal and support of Ni/MgO probably play different roles in gasification that MgO decomposed lignin to reactive intermediates and nickel promoted reaction between intermediates and water to form gases. The yield of methane and carbon dioxide increased with increasing water density but then decreased and leveled out to constant values, which indicates that water density affected the reaction kinetics.