The concentration and temperature dependences of the dc electrical conductivity σ of glasses in the PbF₂-2PbO · B ₂O₃ system is investigated. It is found that, at 20-25 mol % PbF₂, the dependences logσ = f([F]) and E _σ = f([F]) exhibit kinks. The concentration dependence of the true transport number of fluorine ions η_F- is studied using the Tubandt method. It is shown that the introduction of 20-25 mol % PbF ₂ leads to a crossover from the protonic conductivity to the unipolar fluorine ionic conductivity (η_F- = 1.00). The results obtained are discussed in terms of the Myuller theory of the microinhomogeneous glass structure associated with the selective interaction of the components during the synthesis.