The supercritical water gasification of xylose, a model substrate for hemicellulose, was carried out at 400 and 450 °C and at a constant pressure of 25 MPa in the presence of acetic acid using a continuous flow reactor. More specifically, we aimed to compare the reaction rate constants of xylose decomposition in both the presence and absence of acetic acid. Upon the application of a residence time of 0.5–5 s, a xylose concentration of 1.5 wt %, and an acetic acid concentration of 1.5 wt %, we successfully elucidated the effect of acetic acid on each reaction within the reaction network for the first time. In the presence of acetic acid, the retro-aldol reactions and carbon gasification production (i.e., the radical reactions) were suppressed, while the acetic-acid-catalyzed dehydration of xylulose to furfural (i.e., an ionic reaction) was enhanced by 2 orders of magnitude. As such, reaction control through the addition of chemical species to either stabilize ions or react with radicals appears possible.