Solid solutions Ca1x-yLuxGdyF2+x+yfor 10−4≤ x ≤ 2 × 10−2and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu3+-F−xand Gd3+F−inn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd3+tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd3+tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF2crystals very highly doped with small trivalent cations.