The influence of high-temperature annealing (900°C, 14 days) in a fluorinating atmosphere on the superionic conductivity σ of as-grown Sr0.68Pr0.32F2.32 single crystals with fluorite structure (CaF2) and Pr0.85Sr0.15F2.85 single crystals with tysonite structure (LaF3), obtained by the directional solidification of melts in the SrF2-PrF3 system using the Bridgman method, is studied. The annealing that brings the defect structure of crystals to equilibrium at 900°C (which is retained by quenching) does not affect σ values. At 500 K, σ is 4 × 10−4 and 2 × 10−2 S/cm for Sr0.68Pr0.32F2.32 and Pr0.85Sr0.15F2.85, respectively. As-grown superionic conductor single crystals, which are formed in some MF2-RF3 systems (M = Ca, Sr, Ba; R are rare earth elements), where the mutual solubility of components in nonstoichiometric phases depends weakly on temperature, can be used in solid-state electrochemical devices operating at high temperatures and under temperature cycling conditions without the deterioration of their electrical characteristics.