Single crystals of β-alumina doped with Gd 3+ have been obtained by high-frequency induction melting of a NaAlO 2Al 2O 3 mixture and slow cooling, followed by a Gd 3+Na + exchange in molten GdCl 3. In these crystals, Gd 3+ occupies only one out of the several possible sites, in the spinel block and not in the conduction plane (which could be anticipated from the ionic radius of the ion). The EPR data can be described by a spin Hamiltonian which includes trigonal terms. The b20 value is close to 0.08 cm -1. These results suggest a localization of the ion either in the 4 f-layer tetrahedra, or in an interstitial site. The second possibility, which is consistent with charge compensation (due to nonstoichiometry in β-alumina) by interstitial oxygen ions seems more likely.