The preparation and characterization of pure fluorite-type phases allowed exploring the CeO2-ZrO2-PrOx (CZP) phase diagram. On the basis of magnetic susceptibility measurements, the Pr 4+ /Pr 3+ molar ratio of several oxides annealed at T=700°C under air was determined; the higher the Zr content, the lower the Pr 4+ concentration. Thermogravimetric Analysis (TGA) and Temperature Programmed Reduction (TPR) measurements showed the various Pr and Ce reduction steps. The Pr 4+ reduction starts at T=250°C and is maximum around T=400°C. For the most reducible compositions, which exhibit the lowest Zr content and the highest Pr rate, the reduction phenomena strongly depend on the Pr/Ce molar ratio. As a remarkable result, Pr and Ce reduction can simultaneously take place at lower temperature (T>430°C) than for oxides of the CeO2-ZrO2 solid solution evidencing that the increase of Pr content also allows enhancing the reducibility of Ce 4+ at low temperature. On the basis of a discussion taking into account a probability of oxygen surroundings in disordered fluorite networks, and the released oxygen rate in materials after the first reduction step performed at T<500°C, a mapping of the most probable labile oxygen sites in the CZP phase diagram is proposed. In particular, it is shown that for the oxides containing 10% Zr at., the most labile oxygen site should be systematically coordinated with one Zr atom, one Ce and two Pr atoms. In the same series (10% Zr at.), electronic transport properties allowed showing semi-conducting behavior with a strong increase of the total conductivity as the Pr content raises. On the basis of the thermal variation of the Seebeck coefficient, these phenomena are associated to electrons and holes hopping, involving intra-atomic charge transfers which depend on the reduction temperature of Pr 4+ ions under air. Finally, the oxygen mobility strongly increases with the Pr content in this series. The oxygen tracer self-diffusion coefficient D* has been estimated by two independent measurements and the best value is around 10-8 cm 2 .s-1 at T=400°C for the Ce0.45Zr0.1Pr0.45O2-x composition, which is quite high in this temperature range. These fundamental properties of CZP phases design very promising new materials like automotive exhaust catalysts, gas sensors, electrolytes or oxygen electrodes for solid oxide fuel cells. INTRODUCTION. In the last 20 years, intensive efforts have dealt with the environmental impact decrease of the automotive exhaust gases. Cerium-based oxides, deemed as oxygen buffers, have been widely investigated as key components for Three Way Catalysts (TWCs, stoichiometric air-fuel mixture), Diesel soot abatement (Lean-burn conditions) and wet oxidation 1,2,3,4,5. Ceria-Zirconia mixed oxides materials have been developed as TWCs 1. They exhibit high Oxygen Storage Capacity (OSC) with high oxygen release under reducing conditions, fast kinet-ics, high content of reducible Ce 4+ at T<600°C, and high thermal stability with rather good surface areas (between 20 and 40 m 2 /g) sta...