Abstract The luminescence of some rare-earth ions in magnesium fluoroborate, Mg 3 F 3 BO 3 , is reported and discussed. The luminescence spectra of Pr 3+ , Eu 3+ , Gd 3+ and Dy 3+ reveal emissions which, especially in the case of Eu 3+ and Dy 3+ , are different from what is observed usually. The luminescent centre appears to be a rare-earth ion which is associated with an O 2− ion replacing a F − ion in the lattice. The covalent character of the luminescent centre and a strong (linear) crystal-field explain part of the anomalous emission lines in the spectra. The covalency accounts also for the low-energy position of the Eu 3+ -O 2− charge-transfer transition and for the low efficiency of the Eu 3+ luminescence under charge-transfer excitation (20%).