Reversible amorphous to crystalline phase transition introduces high contrast in the optical and electrical properties of chalcogenide glasses. This effect can be utilized by a designated temperature sensor based on optical power measurement as a function of temperature for temperature monitoring. For this purpose, crystallization kinetics and crystal structures of Ge–Se binary chalcogenide glasses were studied with Differential Scanning Calorimetry, Raman spectroscopy, and X-ray diffraction spectroscopy. The refractive index as a function of temperature was also measured to correlate the effect of structural rearrangement at the phase transition point with optical properties. Based on these data, the crystallization process is interpreted as being homogeneous for the stoichiometric composition and heterogeneous for either chalcogenide- or germanium-rich compositions. This specifically affects the optical performance of the films as a function of temperature and suggests the application of chalcogen- or germanium-rich compositions for building the sensor.