A novel technique has been developed for measuring the temperature of the calcium oxide product inside an operating industrial lime kiln. In this technique trace quantifies of a rare earth element are incorporated into the product causing it to exhibit strong fluorescence under ultraviolet excitation. The decay time constant of this fluorescence which is strongly temperature dependent serves as a measure of product ternperature. The spectral and temporal characteristics of the fluorescence of several rare earths have been studied over a temperature range of 800 to 1150C which is the range of interest in an operating kiln. This workhasled totheselectionofpraseodyrnium asthernostsuitable rareearthelementand to the development of techniques to permit temperature measurements to be performed with an accuracy of approximately at rare earth concentrations in the parts per million range. A prototypesensoremployingthisfluorescencetechnique hasbeen used successfully to make measurements of lime temperature in an operating kiln. These measurements are in good agreement with simultaneous thermocouple measurementsoflimetemperatureand providethe firstaccurate in-situ measurementof lime kiln product temperature which can be made on a continuous basis.