Polycrystalline rock can be fragmented and penetrated, when hot supercritical water jets impinge on it. Knowledge about the heat transfer between supercritical water jets and the rock's surface is absolutely crucial for this drilling method called hydrothermal spallation rock drilling. The present work for the first time provides systematic heat transfer data of impinging, turbulent, near- and supercritical water jets confined by a cylindrical wall. The most striking result is the dependence of heat transfer coefficients on the surface temperature of the impingement plate: experiments at supercritical jet temperatures performed with two different calorimeter types show remarkable differences between heat transfer coefficients obtained with low surface temperatures (at high heat fluxes) and high surface temperatures (at low heat fluxes). However, the experimental data of both calorimeters could be incorporated in a single empirical correlation by accounting for the variation of individual fluid properties across the jet's thermal boundary layer. (C) 2013 Elsevier B.V. All rights reserved.