Lignin, cellulose, and their mixture were gasified with a nickel catalyst in supercritical water at 673 K and 25 MPa. Gasification efficiency is low, but increases with the amount of the catalyst when softwood lignin is included in the feedstock. One possible mechanism is the catalyst being deactivated by tarry products from the reaction between cellulose and softwood lignin. Mixtures with hardwood and grass lignin were gasified much more easily. Sufficient amount of catalyst achieves high gasification efficiency even for the mixtures of cellulose and softwood lignin. Sawdust and rice straw were also gasified under the same condition. Interaction between each component was observed also for these real biomass feedstocks. (C) 2003 Elsevier Ltd. All rights reserved.