In this work a custom made sensor with an extended service life at temperatures up to 900 degrees C and able to sustain intermittent temperatures in excess of 1000 degrees C is used to ignite and monitor a hydrothermal combustion at 260 bar. Once calibrated, the sensor features a mean sensitivity of 27 mu V/degrees C and an absolute systematic uncertainty of #10 degrees C with 95% confidence from room temperature to 1000 degrees C. After ignition, the influence of the fuel injection geometry, the equivalence ratio and the fuel mass flow on the internal temperature profile are investigated and in the end, applications of the proprietary sensor in supercritical water oxidation and hydrothermal spallation drilling are highlighted. (C) 2016 Elsevier B.V. All rights reserved.