The luminescent properties of hexagonal have been investigated. For and at room temperature, excitation into the Ce3+ ion results mainly in Tb3+ emission. We also observed two different Ce3+ emissions. This is related to the crystal structure of . It is shown that the excitation energy migrates from the Ce3+ ion over the Gd3+ sublattice to the Tb3+ traps. Energy transfer probabilities for this process are derived and compared with the probabilities derived for the system , which leads to some recommendations for the use of the Ce3+ ion as a sensitizer of the Gd3+ ion.