A new method for the determination of thermocouple time constants is described, which reproduces the large temperature differences between the thermocouple junction and ambient fluid that occur in premixed turbulent flame applications. Time constants measured while the thermocouple is both heating and cooling, as well as the dependency of the time constant on velocity and temperature, are reported. Velocities are varied from 0.6 to 5.5 ms-1, and junction temperatures range from 700K to ISOOK while heating and from 400K to llooK while cooling. Temperature differences between the thermocouple junction and ambient fluid up to 800K are achieved. Radiation losses from the junction are accounted for. Predicted time constants based on commonly used correlations for heat transfer from a cylinder in cross-flow are compared to the data, and it is suggested that free molecular flow effects are partially responsible for the systematic deviation of the data from the theoretical predictions.