a Supercritical water jets are of interest for the hydrothermal spallation drilling technology to thermally fragment rock. The important thermal field of such jets was investigated via axial temperature measurements inside a high-pressure vessel simulating the harsh pressure and temperature conditions found downhole. Deionized water was electrically preheated to supercritical temperatures and afterwards discharged into a slowly co-flowing cooling water ring. Furthermore, a numerical model was developed and applied. Difficulties in simulating the thermal field due to the strongly varying fluid properties around the pseudo-critical point of water were highlighted and critically discussed. An approach of a locally varying turbulent Prandtl number is introduced to improve the predictions compared to the experiment. Finally, all numerical results were validated with experimental data.