In this paper, an experimental investigation is presented that concerns the gasification of glycerol, glucose, and pinewood in supercritical water. The batch experiments were performed in quartz capillary reactors with an internal diameter of only 1 mm. Because these quartz reactors are catalytically inert, the process could be studied in the absence of the interfering catalytic influence of a metal reactor wall, as used in all previous studies. The reaction space has been mapped by performing over 700 experiments in which the temperature, pressure, reaction time, and concentration of the feedstock were varied. The most important observations were that the pressure turned out to have no effect on the conversion and product yields, and that, noncatalytically, complete conversion to the gas phase is only possible for very diluted feedstock solutions (<2 wt %). By adding ruthenium on a TiO2 carrier (Ru/TiO2) to the capillaries, the potential of heterogeneous catalysis has been demonstrated. When adding this catalyst to the capillaries, glucose solutions in the range of 1−17 wt % could be gasified completely.