The critical current density of superconducting rare-earth barium copper oxide (REBCO) thin films is enhanced by adding nanoparticles to generate artificial pinning centers. Barium-based perovskites are popular nanoparticle materials. Although typically considered chemically unreactive in the surrounding REBCO matrix, previous studies have shown experimental evidence for rare-earth element (REE) incorporation from REBCO into the nanoparticles. However, not much is currently known about this chemical interaction. Here we apply analytical electron microscopy to study the mixing of REEs into BaHfO₃(BHO) nanoparticles. We find that the REE mixing into BHO depends on the ionic radius and the REE content of the REBCO phase. We investigate various REBCO films with different REE compositions (single and mixed REEs) and the same BHO nanoparticle concentration produced with chemical solution deposition. REEs with smaller ionic radii are more likely to be incorporated into BHO with a preferential substitution at the Hf site. As a result, REBCO films with a mixed-REE composition show an apparent disparity in REE–BHO intermixing. Our results demonstrate that REE–BHO intermixing is predictable and should be accounted for during REBCO fabrication. These insights are likely to apply to other barium-based perovskite nanoparticles and REBCO fabrication techniques and help to optimize REBCO performance.