Abstract: A method and device to measure parameters of temperature and gas composition in high temperature fluids which has particular application to compressible unsteady flowing fluid and rapidly changing temperatures of high temperature gases. The device comprises at least two temperature sensing elements, means for pre-setting said elements to different temperatures, and means for determining a parameter of the fluid from temperature readings of said sensing elements taken at least two time instants. The invention also provides a method of measuring fluid temperature by determining differing heat transfer rates from the temperature history of temperature sensors initially set to different temperatures before exposure to the fluid. Based on the same methodology and device, gas composition of binary flows can also be determined.