The REBa 2 Cu 3 O 7-x (REBCO/YBCO) based high-temperature superconductors are promising materials for high magnetic field and energy applications due to their effective flux pinning abilities. The superconducting properties of these materials strongly depend on the microstructure, which could be engineered by the fabrication methods, and incorporating various dopants. Infiltration growth (IG) process is currently evolving and superior to the standard melt growth (MG) technique addressing various inherent and unwanted problems. However, the IG technique is complex as compared to conventional MG technique, and for high reproducibility, many parameters need to be optimized for every system. In this paper, we present the engineering of the final microstructure with various nanometric metal oxides to the bulk YBCO/REBCO materials aiming to improve the field dependence of the superconducting properties. The effect of introducing various RE elements in IG processed YBCO/GdBCO bulk superconductors, and the evolution of microstructures supported by thorough elemental analysis will be discussed. Emphasis will be given to the problems involved in the growth of mixed REBCO products in the IG process and to the microstructural properties at various stages of processing the bulk samples.