The formation of fluorite-like solid solutions near composition Ln5Mo3O16+δ is observed in binary systems Ln2O3–MoO3, where Ln=Nd and Pr, at synthesis in the air. The oxide ratio Ln2O3:MoO3 within homogeneous solubility range varies from 3:4 to 7:8 and from 9:11 to 7:8 for neodymium and praseodymium oxides, respectively. The oxygen pressure dependent isotherms in Ln5Mo3O16+δ demonstrate mixed, oxygen-ion and electron, character of conductivity. The ion transference numbers change from 0.16 to 0.92 at variations of oxygen pressure and temperature within 10–5–0.5atm and 700–950°С, respectively. The oxygen ion and electron components of conductivity are characterized by the activation energy of about 1.0 and 1.4eV, respectively. It is argued that the ion contribution is related with jumps of off-stoichiometric oxygen over structural interstitials in the fluorite-like crystalline lattice.