Low-temperature ionic conductivity and dielectric relaxation phenomena in Ba 1 _xLaxF2+x crystals (1.19 X 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 ~tudy of the same crystals at higher temperatures. At low LaF 3 concentrations, i.e. 1.19 X 10 -3 ~ x ~ 2.37 X 10 -2 , the dielectric spectra show three relaxation peaks, two of which are ascribed to simple associates of interstitial F-ions (Fi) nearest-neighbour (nm) and next-nearest-neighbour (nnn) to substitutional La 3÷ ions (LaBa). They are denoted as type I and II dipoles, respectively. We attribute the third peak to angled ("L-shaped") (LaBa2~'i}' complexes. Relaxation parat~etors for the relaxations are presented. We have calculated effective dipole moments for type I . eli eli .and II dipole8 PI and/Zli , taking rote account the displacements and polarizabilities of the defects. Several combinations of host lattice cations and dQpant ions have been co~idered for the fluorites M l -xRExF2+x, i.e. M = Ca, S~Ba, and RE = La, Eu, Lu. For nearly all combinations the ratio ~[f/~ff is found to have a value of 2.4 ± 0.1. With the tt eII 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-Hiickel-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 (AH R) which are ghe same as the corresponding conductivity activation enthalpies (AH), and is ascr~ed to F-interstitial motion in crystal areas around defect clusters. The second process is characterized by AH R values which are much lower than AH. 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 I _xLaxF2+x solid solutions.