Disclosed is a method for the non-invasive optic determination of the temperature of a medium, preferably a water-containing medium, wherein the medium to be analyzed is illuminated by infrared and/or visible light in the region of an absorption line, the position of which depends on the temperature of the medium, and wherein absorption of the light in the region of the absorption line is measured and the temperature is determined from said measurement by comparison with calibration data. Said method is characterized in that the medium is illuminated with at least two discrete light wavelengths (λ1, λ2), which are in the region of the absorption line (B) on different sides of the absorption maximum, that at least one measured value (ΔA/Δλ) dependent on temperature is determined from the relationship of these two determined absorption values to one another, and that the temperature is determined from said measured value by comparison with the previously recorded calibration data.