Spectroscopic measurements of infrared molecular transitions in gas plumes are evaluated for their potential to yield a reliable remote sensing technique for determination of plume temperature. Under optically thin conditions, the ratio of intensities from two different CO2 transitions has no dependence on molecular concentration, but has sufficient thermal sensitivity to be used as a temperature diagnostic. Fine spectral resolution is not required. Experimental results involving simultaneous thermocouple and spectroscopic measurements support the use of spectroscopic intensity ratios to determine stack plume temperatures. Measurements involve a plume from a vehicle exhaust and a stack plume from a propane-burning portable plume generator. Observed CO2 emission near 4.26 µm is affected by optical thickness and by self-absorption of emitted radiation by the cool outer regions of the plume. Measurements on the wings of molecular bands mitigate these absorption effects. Relationships between absorptive and emissive spectral intensities are developed that allow calibration of the temperature diagnostic using readily available absorption spectra. Based on data spanning a range of plume temperatures from 310 to 606 K, the root-mean-square difference between the spectroscopically derived temperatures and thermocouple data is 22 K.