Studies of single crystals of nonstoichiometric fluorides of PrF₃:Sr²⁺ and NdF₃:Ba²⁺ in contact with a Pt electrode were carried out using the method of voltammetry with a linear potential sweep, chronoamperometry, and cyclic voltammetry. The reaction of gaseous oxygen reduction (GOR) at the PrF₃:Sr²⁺/Pt interface was accompanied by appearance of mobile oxygen forms. The catalytic activity of PrF₃:Sr²⁺ with respect to gaseous oxygen, same as CeF₃:Sr²⁺, can be explained by variable valency of praseodymium and cerium fluoride, which creates opportunities for using them in oxygen and oxygen-containing gas sensors. The GOR reaction at the NdF₃:Ba²⁺/Pt interface appeared as a result of anodic polarization. After charging at E = 3.2 V, the NdF₃:Ba²⁺ single crystal was successfully used for fluorination of Sb and Ag. NdF₃:Ba²⁺ is most promising in devices based on reactions with participation of mobile fluoride ion in solid electrolytes, particularly, for generation of gaseous fluorine.