Supercritical water gasification (SCWG) of biomass is one of the most promising ways to convert wet biomass into a valuable gas mainly composed of methane and/or hydrogen, according to the selected experimental conditions. A wide range of biomass can be classified as ligno-cellulosic, meaning that its main components are represented by cellulose and lignin. Such two main constituents of biomass behave in a completely different manner. It is known that lignin is much more difficult to gasify than cellulose. However, the effect of such inhibition has never been studied in great detail. In this work, glucose and phenol are used as model compounds for cellulose and lignin, respectively. Four aqueous mixtures were prepared, with a fixed organics content of 5 wt %, the remaining part being constituted by water. The organics content was represented by glucose/phenol in different relative amounts, ranging from a phenol concentration of 0 wt % (glucose only) to 30 wt %. Such mixtures underwent SCWG in a continuous tubular reactor at 400 °C and 25.0 MPa, with different residence times, ranging from 10 to 240 s. Results show that phenol gives a limited contribution to gas production. On the other hand, phenol does not seem to be an inert, since it reacts in the liquid phase to form other compounds, probably tar and/or solids.