Abstract <p>Upon an interaction of protoporphyrin IX free base with an excess of vanadyl sulfate in refluxing dimethylformamide, corresponding vanadyl protoporphyrin is obtained in a high yield. The structure of vanadyl protoporphyrin is determined by the data of EAS and mass spectrometry. An analysis of the temperature dependence of the phosphorescence spectra of a polystyrene film including vanadyl protoporphyrin is conducted upon thermostating using a cryostat. It is found that, throughout the entire range of temperatures of from –195 to 25°C, the ratio of the intensity of transitions of phosphorescence ⁴<i>T</i>₁–²<i>S</i>₀ and ²<i>T</i>₁–²<i>S</i>₀ changes gradually and completely reversibly without hysteresis effects. An analysis technique of phosphorescence spectra of vanadyl protoporphyrin for the determination of temperature by the shape of a phosphorescence spectrum that contains one pronounced maximum with a set of shoulders in the red visible region is proposed.</p>