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Abstract Multicomponent oxides have received significant recent attention due to their potential for improved property tunability. In simple structures, compositionally complex oxides can be stabilized by increased configurational entropy and are sometimes called “high entropy” ceramics. In phases with multiple cation sublattices or complex stoichiometries, it is more difficult to achieve high configurational entropy. However, there is limited knowledge about the factors influencing stability and solubility limits in many systems. This study investigated the limits on the stability of rare earth (RE) aluminates containing mixtures of RE cations including Gd, La, Nd, Yb, and Y in cases where (i) a fixed RE:Al ratio attempts to constrain the material into a single‐phase aluminate or (ii) a two‐phase aluminate, and in equilibrium with RE zirconates that readily dissolve multiple RE³⁺. The results show that it is difficult to form single‐phase, equimolar mixed‐RE aluminates encompassing a range of RE³⁺sizes. Instead, the RE³⁺selectively partition into specific phases based on RE‐size trends in the constituent binary systems. The results are discussed in terms of the phase stability and cation partition trends and potential applications.