The formation of a thin layer, the so-called Joint Oxyde-Gaine (JOG), between the (U,Pu)O 2 fuel pellets and the cladding has been observed in fast neutron reactors, due to the accumulation of volatile fission products. Cs 2 MoO 4 is known to be one of the major components of the JOG, but other elements are also present, in particular tellurium and palladium. In this work, an investigation of the structural and thermodynamic properties of Cs 2 TeO 4 and Cs 2 Mo 1– x Te x O 4 solid solution is reported. The existence of a complete solubility between Cs 2 MoO 4 and Cs 2 TeO 4 is demonstrated, combining X-ray diffraction (XRD), neutron diffraction (ND), and X-ray absorption spectroscopy (XAS) results. High-temperature XRD measurements were moreover performed on Cs 2 TeO 4 , which revealed the existence of a α–β phase transition around 712 K. Thermal expansion coefficients were also obtained from these data. Finally, phase equilibra points in the Cs 2 MoO 4 –Cs 2 TeO 4 pseudobinary phase diagram were collected using differential scanning calorimetry and used to develop a thermodynamic model for this system using a regular solution formalism.