In hydrothermal spallation deep drilling a high-velocity, hot, supercritical water jet is directed towards the rock to induce fragmentation. One major challenge in the realization of this novel technique is the entrainment of comparatively cool, aqueous drilling fluid by the hot water jet, which can lead to significant heat losses before the hot jet's energy can be transferred to the rock. The present work quantifies such entrainment effects by determining penetration lengths of supercritical water jets injected into a cool, subcritical environment using a novel optical Schlieren method. Penetration lengths of supercritical jets were found to be equal to the injector's nozzle diameter and almost independent of the jet's temperature at the nozzle exit and the jet's mass flow under almost all experimental conditions investigated. A semi-empirical model adapted from steam jet studies confirmed these findings and indicates that heat and mass transfer are primarily controlled by turbulent mixing. (C) 2011 Elsevier B.V. All rights reserved.