A sensitive and broad-ranged optical thermometer, based on the thermally activated delayed fluorescence of fullerene C70, is presented. It consists of C70 molecularly dispersed in a polymer film. Several polymer matrices were investigated. In the absence of oxygen the fluorescence intensity increases markedly with temperature. At 25 degrees C the fluorescence intensity of C70 increases maximally by a factor of between 17 and 22, depending on the polymer, whereas at 100 degrees C the fluorescence intensity can be 79 times higher. In the absence of oxygen and for temperatures above 20 degrees C, the red fluorescence of C70 in the films is so intense that it is easily perceived by the naked eye. For the systems studied, the fluorescence intensity is very sensitive to temperature. This results in a working range from -80 to at least 140 degrees C in the case of C70 in poly(tert-butyl methacrylate) (PtBMA). Perylene was incorporated into the film as an internal reference in order to enable ratiometric measurements. The sensitivity of the lifetime of the delayed fluorescence to temperature is also high and results in an even wider working range. The performance of the C70/PtBMA film was measured against a well-known optical temperature probe, [Ru(phen)3] (phen=phenanthroline). The results show that the C70/PtBMA film is a very good system for optical temperature-sensing over a wide range of temperatures, outperforming known standards.