This paper discusses the calibration and use of the Thermo-Systems Inc. aspirating probe in a particular application where temperatures of a hot gas plume are estimated from measurements on a dynamically similar helium—air plume. The probe is used as a constant-temperature hot-film anemometer but, being enclosed in series with a choked orifice, it responds primarily to variations in gas conductivity and temperature. T theoretical analysis of the response of the instrument in a helium—air mixture shows that a normalised calibration curve can be found, in which temperature effects are isolated at the all-helium and all-air reference points. The experimental calibration curves are similar to the theoretical form. The aspiring probe has poor resolution compared with more conventional gas-sampling instruments and is therefore not sufficiently sensitive for measurements of chemical pollution. Nevertheless, in the present application it is capable of resolving temperature differences of 0.3°C with a high frequency response and is therefore a valuable tool in the exploration of plume trajectories near the source in a turbulent wind.