The primary aim of this study was to investigate the use of hydrothermal jets as a means of drilling rock. Specifically, an electrically heated hydrothermal jet was impinged on the surface of cylindrical Barre Granite samples (basement rock) contained in an autoclave reactor to induce thermal spallation. Comminution of the rock surfaces was achieved at supercritical water conditions, temperatures from 535- 580°C and pressures of 22.5-27MPa. These conditions simulate those encountered in drilling deep, water-filled wells. Preferential removal of quartz grains from the rock matrix was observed. However, experimentally determined heat flux and surface temperature measurements indicated that hydrothermal comminution occurred below the empirically determined minimums for the onset of continuous thermal spallation for low-density flame jet drilling. Chemically enhanced hydrothermal spallation drilling experiments introduced sodium hydroxide into the hydrothermal jet, at the same supercritical conditions. The combination of accelerated mineral dissolution due to the presence of hydroxide ions, high temperature kinetics, and thermal stress induced significant rates of rock removal. Comparison with empirical quartz dissolution data indicated this drilling success was primarily due to chemical dissolution, enabling the quantitative modeling of an effective drilling rate.