Effect of organic acids on char suppression in supercritical water gasification (SCWG) was investigated. When acetic acid or formic acid was added to the SCWG of guaiacol, char production was suppressed. Acetic acid also effectively suppressed char formation in the SCWG of a Shochu residue. When an optimal amount of acetic acid was added, char production was drastically suppressed, while hydrogen was produced in excess of the added acetic acid. Further addition of acetic acid increased the char production as well as the production of methane and carbon dioxide. We believe that the organic acids function as radical scavengers to suppress char formation that usually occurs via a radical mechanism.