Supercritical water gasification (SCWG) of mixtures of guaiacol and acetic acid was carried out in a continuous reactor at 600 °C and 25 MPa with a residence time of 94 s. Different concentrations of acetic acid were employed to investigate the effect of acetic acid on product yield and gas composition. The interaction between guaiacol and acetic acid during SCWG was discussed. Acetic acid, as a radical scavenger, was found to inhibit radical polymerization, resulting in the suppression of char formation.