In this work, we explore the high-temperature phase stability of isolated, alumina-coated zirconia nanocrystals with a goal of understanding how interfacial energy affects phase stability. Isolated tetragonal and hydrous amorphous zirconia colloids were synthesized and coated with alumina through the hydrolysis of aluminum isopropoxide. Alumina-coated samples exhibited phase behavior that was markedly different from that of the uncoated analogs. Uncoated tetragonal particles transformed to the monoclinic phase at 1100 degreesC while alumina-coated tetragonal particles did not transform until 1400 degreesC. Uncoated hydrous amorphous particles crystallized to the tetragonal phase after heating at 600 degreesC and transformed to the monoclinic phase after heating at 800 degreesC. Alumina-coated hydrous amorphous particles crystallized only after heating at 1050 degreesC, and transformed to the monoclinic phase after heating at 1400 degreesC. Differences in phase behavior are postulated to depend on the zirconia-alumina interface, which must be disrupted before zirconia particles can fuse and facilitate the tetragonal-to-monoclinic phase transition. By coating the nanocrystals with a thin alumina shell and studying the resultant phase stability, we explore the effect of reproducibly modified interfacial chemistry on phase behavior in nanoscale ceramic composites.