To study the different populations of defects existing in Ba1-xGdxF2+xsolid solutions forxranging from 0.0001 to 0.1, comparative Ionic Thermal Currents (ITC) and Electron Paramagnetic Resonance (EPR) experiments have been performed. In the as-grown crystals weak trigonal centers assigned to the nnn Gd3+-Fi−dipoles coexist at all concentrations with Gd3+in cubic sites. The variation of the number of these nnn dipoles as a function ofxfound with the two series of experiments is a strong indication of the existence of larger clusters in the crystals. The appearance of new relaxation peaks in the low temperature zone of the ITC spectra is correlated to the formation of these aggregates together with the observed variation in the number of cubic sites. An association energy of 0.41 eV of the nnn dipole is calculated from the equilibrium of associated and free defects. The less doped crystals were treated thermally at 1223 K and variable amounts of O2−ions diffused into the samples creating Gd3+and O2−centers in which the oxygen to gadolinium ratio varies. It is shown that the trigonal T2center is the first step in the oxidation process and at least one new O2−center is shown to exist in an annealed crystal where no trigonal or cubic Gd3+centers exist. The structure proposed for this center is consistent with the appearance of a new relaxation peak in the ITC spectrum obtained after the annealing. The Monte Carlo procedure for fitting the effective spin Hamiltonian parameters is used for identifying the weak trigonal, the cubic and the T2Gd3+centers.