The accurate determination of the defect structure of rare earth (RE) doped ceria solid electrolytes is fundamental for the comprehension of oxygen diffusion and charge transport within these materials. The mismatch between Ce4+ and RE3+ ionic radii is supposed to favour the oxygen vacancies and dopant ions clustering limiting the oxygen mobility. To minimize this effect it has been proposed to vary the ?average? ionic radius of RE by a mixture of large and small cation like e.g. La3+ and Yb3+.The aim of the experiment is to clarify the compositional evolution of defect clustering and the oxygen vacancies ordering in RE double-doped ceria. We propose to investigate the Ce1-x-yLaxYbyO2-(x+y)/2 solid solutions by means of PDF technique because it allows to unveil both the local (within few Å) and the mesoscopic (within few nm) disorder induced by doping in these solid solution.