Temperature dependence of the intensity of the phonon-assisted excitation probability (as a function of the intensity of the luminescence bands corresponding to the (Formula presented) transitions) of (Formula presented) ion in GdTb((Formula presented) (GTMO), above and below the ferroelectric phase transition temperature ((Formula presented)) is investigated in the present work. The experiments reveal a sharp change in the intensity near (Formula presented) . This change is attributed to a sharp change in the electron-phonon coupling near (Formula presented) . A phenomenological model is proposed to fit the experimental data, in broad accordance with the Ginzburg-Landau equations for critical behavior. The investigations also clearly indicate that phonon-assisted excitation probability can be used to probe structural phase transitions. © 1997 The American Physical Society.