Abstract: In this paper, the mixed oxides of non-radioactive lanthanides were prepared in order to simulate the mixed oxides of minor actinides, as their chemical properties are similar. CeO₂-based mixed oxides Ce_1-xLn_xO_2-x/2 with Ln=Nd or Sm and x∈［0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1］ were prepared through calcination of the oxalate precursors (Ce_1-xLn_x)₂(C₂O₄)₃•10H₂O elaborated by coprecipitation method which led to a homogeneous distribution of cations in the mixed oxides. For the characterization, the infrared spectroscopy, Raman spectroscopy, X-ray diffraction and thermo-gravimetric analysis were applied for the oxalate precursors. Then investigations of the crystalline structure of these oxides Ce_1-xLn_xO_2-x/2 were carried out by X-ray diffraction. Typical fluorite Fm-3m structure is observed for relatively low Ln³⁺ contents (x≤0.4 for Nd and x≤0.3 for Sm), then a cubic Ia-3 structure is observed (x≥0.5 for Nd and x≥0.4 for Sm). In addition, the Rietveld method was used for refining the unit cell parameters of Ce_1-xLn_xO_2-x/2. The variation of the unit cell parameters in different series is found to follow a quadratic relation as a result of combination between increasing cationic radius and decreasing O-O repulsion caused by oxygen vacancies.