We report on the use of thermoluminescence and radioluminescence to monitor first order
phase transitions. The luminescence signals not only offer information on the mechanisms of
the relaxation processes, including changes in charge trapping energies and emission
spectra, but also provide unique, sharply defined, temperature signals to act as temperature
calibration standards for thermoluminescence. The wider implications of the method are
mentioned, including the inverse problem of detection of phase transitions.