Corundum, alpha-Al2O3, is the thermodynamically stable phase of coarsely crystalline aluminum oxide, but syntheses of nanocrystalline Al2O3 usually result in gamma-Al2O3. Adsorption microcalorimetry, thermogravimetric analyses, and Brunauer-Emmett-Teller adsorption experiments, coupled with recently reported high-temperature solution calorimetry data, prove that gamma-Al2O3 has a lower surface energy than alpha-Al2O3 and becomes energetically stable at surface areas greater than 125 square meters per gram and thermodynamically stable at even smaller surface areas (for example, 75 square meters per gram at 800 kelvin). The results are in agreement with recent molecular dynamics simulations and provide conclusive experimental evidence that differences in surface energy can favor the formation of a particular polymorph.