A set of nine samples with nominal composition Ce0.5RE0.5O1.75 (RE = Dy-Lu, Sm, Gd and Y) has been used to develop a novel method based on Raman spectroscopy, useful to evaluate the RE coordination number within the fluorite-type structure (F) of the CeO2-based solid solution. The RE coordination number is a crucial indicator of defects aggregation in doped ceria, and thus of the drop in ionic conductivity observed even well below the compositional limit of the F structure. The method has been tested on several Ce1 - xRExO2 (-) (x/2) compositions (RE = Sm, Gd and Lu, 0.1 <= x <= 0.4); the obtained results are compared to literature data deriving from EXAFS spectroscopy, and advantages and criticisms of the method are thoroughly discussed. Samples were prepared by a coprecipitation method, their cationic composition was checked by SEM-EDS and their structural features were analyzed by room temperature synchrotron powder x-ray diffraction.