The extent of the relationship between rare earth radius, peritectic decomposition temperature (Tp), and lattice dimensions within mixed rare earth (Nd, Eu, Gd) barium cuprate 123 superconductors has been investigated. The peritectic decomposition temperature of compositions within the “ternary” (NdxEuyGdz)Ba2Cu3O7 − δ (x + y + z = 1) material, while maintaining a constant average rare earth cationic radius of 1.076 Å, was observed to be uniform. Powder X-ray diffraction analysis of processed materials also showed the lattice parameters to be uniform with these compositional changes. These results indicate a uniform mixing of the RE elements in the RE 123 phase. In contrast, the mixing of such RE elements leads to the formation of an inhomogeneous distribution of both Nd422 and RE211 second-phase particles in the matrix subsequent to melt processing.