A method and apparatus for measuring the temperature of a liquid medium are disclosed. In accordance with the present invention, a liquid medium containing a fluorescent dye is provided. The fluorescent dye is chosen to exhibit a first fluorescence intensity at a first wavelength and a second fluorescence intensity at a second wavelength, wherein the temperature of the liquid medium may be determined in accordance with a predetermined temperature function of the first fluorescence intensity and the second fluorescence intensity. The method of the invention comprises the steps of measuring the fluorescence intensities at the first and second wavelengths; and determining the temperature of the liquid medium in accordance with the predetermined temperature function. The apparatus comprises means for measuring the first and second fluorescence intensities, and means for determining the temperature of the liquid medium in accordance with the predetermined temperature function. The apparatus preferably is a confocal optical measuring device, and preferably is capable of determining a temperature profile within the liquid medium.