The structural, microstructural, Raman and ionic conductivity properties of (Gd,Sm)-doped ceria were studied and compared to the ones of similar ceria systems with the aim of deepening the comprehension of the correlations between defect chemistry and movement of oxygen vacancies in such materials, which are ideal candidates as electrolytes in solid oxide cells. The system was chosen as it combines the advantages of using the most effective doping ions for ceria, namely Sm3+ and Gd3+, and the expected positive effects of multiple doping. The main effect of double doping on the structure is the enlargement of the compositional region where ionic conductivity takes place, due to the entrance of the smaller doping ions into defect clusters, mainly trimers and dimers (RE ≡ rare earth). On the other hand, the formation of such clusters also affects ionic conductivity, as it causes the occurrence of a double activation energy with a temperature threshold located at ∼770 K. The dissociation of trimers above this temperature induces the appearance of a high temperature activation energy which is lower than the one observed in singly-doped systems, such as Sm- and Nd-doped ceria, showing the unique value of this parameter.