The effect of sulfur on the reaction pathway for lignin gasification was studied over a titania-supported ruthenium catalyst in supercritical water. In the absence of sulfur, lignin was completely gasified to methane, carbon dioxide, and hydrogen over the titania-supported ruthenium catalyst in supercritical water. In the presence of sulfur, the overall gas yield decreased and THF-insoluble products (namely, char) were formed. Also, the content of hydrogen increased in the resulting gas composition. To investigate the poisoning effect of sulfur for lignin gasification, the gasification behavior of 4-propylphenol and formaldehyde, which are low-molecular-weight model compounds from lignin decomposition, was studied in supercritical water. 4-Propylphenol was completely gasified in the absence of sulfur; however, it was hardly gasified in the presence of sulfur. On the other hand, the gasification of formaldehyde proceeded smoothly over the ruthenium catalyst regardless of the presence of sulfur. Also, the content of hydrogen in the gaseous products increased in the presence of sulfur. We concluded that sulfur poisoned the active sites for carbon-carbon bond breaking and the methanation reaction; on the other hand, it did not hinder the sites for the gasification of formaldehyde and the water-gas shift reaction.