The energy of Cr4+local modes and the energy and dynamics of Mg2SiO4phonon modes involved in the nonradiative relaxation of photoexcited impurity Cr4+ions in forsterite crystal were measured. The experimental results suggest that resonance in energy between local vibrations of the impurity ion and lattice (phonon) modes of the host crystal plays a crucial role in selecting the phonons which participate in the nonradiative relaxation of the photoexcited ions in crystals. ©1996 American Institute of Physics.