This paper describes the theory, design, development and usage of a high-speed, aspirated, choked-nozzle, hot-wire concentration probe. This probe, used in conjunction with a detectable foreign gas (FG) coolant, has been applied to the study of a gas turbine film-cooled nozzle guide vane (NGV) under simulated engine Mach numbers and Reynolds numbers.The FG mixture (30.2% sulfur hexafluoride and 69.8% argon by mass) is used in the tests to simulate the engine coolant to mainstream density ratio. This probe is designed to measure the FG concentration at a frequency of up to 200 Hz and with an uncertainty of ±2%. An example of coolant concentrations, downstream of the NGV row, extending across one and a half NGV passages, is presented in the paper to demonstrate the successful design of this high-speed concentration probe.