Single crystals of barium fluoride doped with different rare earth ions (Tb-Sm-Nd-Pr-Dy-Ho-Er) have been grown in a He atmosphere. The far infrared (FIR) spectra observed at 1.4 K show common features at 75.5-125-131 and 156 cm −1. They are explained in terms of localised vibration of the compensation F − ion in a cubic site ( v̇ c = 131 cm −1), a tetragonal one ( v̇ L = 75.5 cm −1 ; v̇ T = 156 cm −1) and a trigonal one ( v̇ L = 125 cm −1). The different frequencie can be calculated in the assumption of a cubic harmonic oscillator ( v̇ c = 131 cm −1) with Coulomb perturbations due to the RE ion at different positions. Special features are observed at 41.2 cm −1 for BaF 2: Dy +3; 37 cm −1 and 50.4 cm −1 for BaF 2: Ho 3+; 69.2 and 70.3 cm −1 for BaF 2: Er 3+. They arise from electronic transitions. The Zeeman effect gives information on the Landé g factors of both ground and excited levels with magnetic fields smaller than 50 kG.