In this paper, the influence of a fixed water mist firefighting system on a high-speed non-premixed horizontal methane jet fire is investigated with focus on its effect on temperatures and heat load of the jet fire and on its surroundings. Six tests are performed in which gas is released out of an orifice with a diameter of 1 mm and a release pressure of up to 186 bar. In addition to temperature and radiation measurements, the release pressure, gas mass flow rate and exhaust gas concentrations are detected to determine the heat release. Video and IR-analysis are used to evaluate the interactions between jet fire and water mist. The experiments show, that water mist reduces the temperatures inside the jet fire flow field and its radiative heat flux. It can lower the risk of ignition of adjacent surfaces and materials, as temperatures fall below autoignition temperatures of common materials like plastics. Although water mist does not extinguish the fire, it has an impact on the energy release by reducing combustion efficiency due to heat exchange with the water mist and oxygen displacement.