We have investigated the dynamics of phonon assisted electronic energy transfer between Eu 3+ ions, experiencing different crystal fields in an amorphous Ca(PO 3) 2 matrix, using the technique of time-resolved fluorescence line-narrowing (TRFLN). From the concentration and temporal dependence of the transfer rate, the mechanism of the ion—ion electronic coupling has been identified as electric dipole—dipole. Both the excitation wavelength independence and the power law temperature dependence of the transfer rate identify the mechanism of the phonon participation in the transfer process as one involving a two-phonon process, with one-phonon action at each of two coupled ion sites.