Gasification of alkylphenols as lignin model compounds was examined in the presence of supported noble metal catalysts in supercritical water without hydrogen donor at 673 K. The activity of the catalyst was in the order of Ru/gamma-alumina > Ru/carbon, Rh/carbon > Pt/gamma-alumina, Pd/carbon, and Pd/gamma-alumina. The effect of water density with the Ru/gamma-alumina catalyst was examined in detail. The main gas products were methane, carbon dioxide, and hydrogen. The yield of gases and the ratio of methane increased with increasing water density. The gasification of the isomers of propylphenols was also examined with the Ru/gamma-alumina catalyst. The reactivities of o- and p-propylphenols were relatively higher than those of m-propylphenols.