Aluminate and zirconate materials containing multiple rare-earth (RE) cations are of interest for next-generation thermal and environmental barrier coatings (T/EBC) due to lower thermal conductivity and tunable chemical response to corrosive species. Many reports focus on the ability to entropically stabilize certain crystal structures (e.g., garnet, perovskite, fluorite) with a various RE cation combinations, without directly assessing changes in the intrinsic lattice stability. To address this need, this study investigated the stability of aluminates containing mixtures of Gd, La, Nd, Y, and Yb either constrained to a single phase based on stoichiometry, or as part of multi-phase assemblages. The results show that at although single phase, mixed-RE aluminates can be stabilized, there is often a strong tendency for selective RE cation partitioning based on RE-size trends in the constituent binary systems. These results provided valuable insight into the relative thermodynamic stability of multi-RE phases to guide next-generation T/EBC design.