Low-temperature ionic conductivity and dielectric relaxation phenomena in Ba 1- x La x F 2+ x crystals (1.19 × 10 -3 ⩽ x ⩽ 0.492) have been studied by ionic thermal current (ITC) and ac dielectric loss (DL) techniques. The conductivity results agree with those of an earlier study of the same crystals at higher temperatures. At low LaF 3 concentrations, i.e. 1.19 × 10 -3 ⩽ x ⩽ 2.37 × 10 -2, the dielectric spectra show three relaxation peaks, two of which are ascribed to simple associates of interstitial F - ions (F ' 1) nearest-neighbour (nm) and next-nearest-neighbour (nnn) to subtitutional La 3+ ions (La Ba). They are denoted as type I and II dipoles, respectively. We attribute the third peak to angled (“L-shaped”) { La Ba 2 F i } ' complexes. Relaxation parameters for the relaxations are presented. We have calculated effective dipole moments for type I and II dipoles μ eff I and μ eff II, taking into account the displacements and polarizabilities of the defects. Several combinations of host lattice cations and dopant ions have been considered for the fluorites M 1- x RE x F 2+ x , i.e. M = Ca, Sr, Ba, and RE = La, Eu, Lu. For nearly all combinations the ratio μ eff II/ eff II is found to have a value of 2.4 ± 0.1. With the μ eff values. thus obtained, the dipole concentrations of type I and II dipoles can be calculated. Their dependence on solute content is discussed. In the calculations, Debye-Hückel-Lidiard interactions between defects and the charge clouds surrounding them have been taken into account. At high concentrations, i.e. 0.133 ⩽ x ⩽ 0.492, broad structureless absorption losses dominate the dielectric relaxation spectra. These losses are analysed by the fractional polarization (FP) technique, and by a special analysis of the DL data. The feasibility of these techniques is discussed. The analysis reveals that two localized relaxation processes occur in concentrated solid solutions. One is characterized by reorientation activation enthalpies (Δ H R) which are the same as the corresponding conductivity activation enthalpies (Δ H), and is ascribed to F-interstitial motion in crystal areas around defect clusters. The second process is characterized by Δ H R values which are much lower than Δ H. We propose that this process is related to F-interstitial reorientation within clusters. The results presented indicate that no typical polarizable cluster is preferred in concentrated Ba 1- x La xF 2+ x solid solutions.