A careful analysis of the impedance spectra of PbF2:SiO2 and PbF2:Al2O3 composites reveals that the increased conductivity of the composites is due to enhanced fluorine ion vacancy concentration. The combination of homogeneous and heterogeneous doping is consistent with the above finding. These results are in reasonable agreement with the "model of heterogeneous doping." Space charge mechanisms as well as various other processes that may contribute to the enhanced conductivity are discussed.