Crystals of RbEu 3F 10 are cubic with a = 11.844 Å, Z = 8, and three possible space groups, Fm3m, F overline43m , and F432. The structure has been resolved from three-dimensional X-ray data and refined by the least-squares method. The final R values are, respectively, 0.065 and 0.067 for Fm3 m and F overline43m , and are not significantly different. The rare earth symmetry is C4 v for Fm3 m and C2 v for F overline43m . Consequently, we have used the Eu 3+ ion in the RbEu 3F 10 phase as a structural probe in order to state precisely the symmetry of the lanthanide site and distinguish among the possible space groups.