Heterovalent isomorphous replacement is one of the main methods to increase the ionic conductivity of solid electrolytes. Understanding the nature of these impurity defects and their relaxation can help in establishing the connection between the electrophysical properties and crystallochemical peculiarities of doped superionic conductors. EPR spectra of Gd3+ in PbF2 show a well resolved fine structure as well as spin forbidden transitions with Delta M(s)> 1. The angular dependence in both cases indicates that Gd3+ occupies a purely cubic symmetric site. This stands in contrast to earlier results and is indicative of the importance of the history of a sample, especially its thermal and pressure treatment, as shown by X-ray analysis and conductivity data. A method of investigating dipole defects and polarizing phenomena is proposed, whereby RE-F-i dipole defects are fixed and their relaxation is studied by means of EPR-spectroscopy.