One approach in hydrothermal spallation drilling is the use of hot, impinging, supercritical water jets to induce rock fragmentation. The heat transfer from hot jets to the rock's surface is of major interest for the drilling process. This work therefore presents heat flux measurements in the challenging environment of confined supercritical water jets impinging on a flat surface. A sensor based on the calorimetric measurement principle and the according experimental methods were developed to determine heat transfer coefficients of subcritical and supercritical water jets. In the subcritical temperature range the results show good agreement with existing empirical correlations originally developed for isothermal, subcritical, liquid jets. An interesting behavior was though observed for supercritical injection temperatures: when the impinged surface is still at subcritical temperatures, the peak of the water's specific heat at the pseudo-critical temperature dominates the heat transfer even at injection temperatures far beyond the pseudo-critical temperature. (C) 2012 Elsevier B.V. All rights reserved.