The spectroscopic properties of Fe3+ in tetrahedral d-sites of GGG and the effect of co-doping with ~m 3 + are investigated. It is thus shown that a very efficient energy transfer from ~e 3 + (d) to ~m~+ takes place due to a good superposition of Fe emission and Tm 3~6 -> 3~4 absorption spectra and due to a favorable packing of the d-sites around the dodecahedral site occupied by Tm. It was also found that the Fe-sensitized luminescence of Tm has different spectral and temporal characteristics from the normal emission of this ion in GGG.