Mixed REBa2Cu3O7-delta bulk superconductors are among the most complex; this complexity is revealed only with sophisticated methods of synthesis of doping, non-superconducting phases or particles and characterization. The changes of their structural properties due to doping results changes in the superconducting properties. The effect of nanometric Nb2O5 and Sm2O3 particles doping on the physical properties of Preform Optimized Infiltration Growth processed YBa2Cu3O7-delta superconducting samples was studied. Structural, microstructural and elemental properties are correlated with the undoped sample. The structural analysis indicated that the doping of Nb2O5 resulted in additional phases. Microstructural features of the doped samples show the presence of different annuli cores in the precipitates and macro-porosity is observed in all the samples. Elemental analysis confirmed the compositions of different phases present in the composites. (C) 2016 Elsevier B.V. All rights reserved.