Measuredvaluesofcombustionefe ciencyc,whichquantifytheamountofhydrogenfuelthatremainsunburned because the fuel has insufe cient residence time in the reaction zone of a supersonic e ame, are reported. Trends are reported as the fuel e ow rate and the stagnation temperature are systematically varied. The combustion efe ciency measurementsareneededtoassesschemistrysubmodelsofnumericalsimulationsofsupersonice ames.Ahydrogen jet e ameisstabilized on theaxis of a Mach 2.5 windtunnel,andto explain why somefuel remainsunburned images were obtained of the fuel concentration and the OH radical concentration using planar laser-induced e uorescence. Increasing the fuel e ow rate (and overall fuel-air equivalence ratio from 0.034 to 0.068 ) is found to increase the combustion efe ciency and the e ame length. It is believed that the increased residence time of fuel in the longer e ames causes the observed increase in efe ciency. The increase the combustion efe ciency caused by an increase in thestagnation temperatureis quantie ed. Obliqueshock waves wereadded and werefound to decrease combustion efe ciency if the waves are positioned to create a radial oute ow of fuel away from the OH radical zone, which reduces the residence time.