A temperature-sensing scheme that relies on kinetically trapping molecular mixtures of a sensor molecule and amorphous host materials in a thermodynamically unstable state is introduced. Subjecting the blends to temperatures above their glass transition leads to irreversible changes of their photoluminescence emission spectra due to phase separation and excimer formation, as shown in the Figure for blend films before (left) and after annealing at 150 Degrees C for 42 h (right).