Batch reactors made of stainless steel and Inconel® 625 were used for the hydrothermal gasification of glucose and beech sawdust under both subcritical (350 °C) and supercritical (400 °C) conditions at a pressure of 30 MPa. These tests were executed over residence times ranging from 60 to 300 min. The amounts of solid, liquid and gas produced and the composition of the resulting gas phase were measured. The results showed that a higher H2 output was achieved in the stainless steel reactors, while the Inconel® 625 reactors were more effective for the synthesis of CH4 and light hydrocarbons. A visual observation of the metal surfaces was performed using scanning electron microscopy to explain the persistence of catalytic activity even after an aging treatment and many hours of operation.